Here is an attempt to link a bunch of theories I like and think are really believable, to make one giant compilation of a theory, explaining everything. Featuring the following theories that I will be referring to by their number:

1. u/diggory_wood's Duplicity theory and u/DecafGrizzly's rotation and windmill theory that are basically two ways of seeing the same thing (which is ironic considering what the theory is about).
2. u/perpetuallyawake's theory saying that a failed rebirthing gives you an egg..
3. u/igordogsockpuppet's theory that the disappearing windmill could be a metaphor for a car accident.
4. u/GonerBits's theory that Tiara actually was Anna and Marvin's dead daughter, one Tiara Leskowitz.
5. u/qorgle's speculation that Tiara could have been Marvin's mother. And I'm going to say that she possibly could have been his sister or some other relative, either from the same generation or one older. And this theory is not contradicting the previous one, you'll see.
6. My own theory of what the family tree most likely is, all theories considered, especially u/WoozieV6's Paul is the third brother theory.

Okay so that's a lot. And some of them are in deep speculation territory, I'm well aware of that. But I couldn't help but find ideas that kinda reappear over some of them and I just love the idea of joining a bunch of theories to make a full story. I'll even add some of mine in the mix in a feeble attempt to make sense of all of this. Here we go.

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The big overarching theory : There Are Two Different States of the Same Things (theory 1).

aka, Paul's reality and Care's. The theories I've linked are wonderful and tell you all the reasons why this could be what's happening, but long story short : Paul and Care are basically the same people but on two different "dimensions", that are broadly identical but for a few details. Everyone (almost) is unaware of the other dimension but Marvin, Rainer (that I am going to say is Daniel, and Jill's son) and possibly Anna. In what follows I will use indifferently "states", "dimensions" and "realities" to describe this duplicity.

These two states are separated but can still communicate : like in the Graverobber game, when in one state, you can still feel the presence of objects present in the other state but can only truly see the ones from your state. And through some mechanism that I can't really think of right now, one could potentially "travel" from one state to the other, whether intentionally or not. Actually, we know that only a few people have.

• Rainer, via the mirror in the Marks bathroom, which would explain the weird "check your bathroom now" and "keep playing the game" while he's in there. Could also mean that he killed himself in there but I think the "travel from one reality to the other" also works.
• Paul and Care, who "exchanged" states during "strange situation" (go read diggory_wood's theory, it's excellent and very detailed).
• Potentially Mike, judging by Anna's "not every one can see [Lina]" quote.
• And Lina herself...

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The beginning : what happened to Lina.

With this "reality rotation" idea in mind, we can talk about what happened to Lina. I'm going to go with theory 3 : Marvin, Lina and Anna were young kids at the time, and the windmill story could have been the visualisation of a car spokes through the eyes of children. So as Lina was hit by a car, she somehow rotated at the same time (I'm not really sure how spontaneous reality rotations can occur but oh well). She literally disappeared into thin air, becoming invisible, as the car hit her. And I'm even going to say that the event was an accident. Why? Remember in the end of Petscop 11 when Marvin is teaching Paul how to play the needles and Paul begins playing it wrong? "She Tripped And Fell And Is Lost". Okay I agree that this could literally mean anything about literally anyone, but since we know that playing the needles is an important aspect of the rebirthing process and that Marvin is desperately trying to make Lina come back through rebirthing, it doesn't seem that far-fetched to say that he is just saying how Lina disappeared in the first place. So Lina tripped and fell, was hit by a car and was lost from her original reality.

She could have lived after that or died immediately after rotating, we don't have much proof of what happened afterwards (as with a majority of these theories anyway), but the thing is she disappeared after the car accident. This explains why the windmill is gone in both realities (because it's just the wheel of a car and the car just went away/wasn't even there in the other reality, Paul's), the "they didn't see her" epitaph, why there is a big tire above Lina's unmarked grave, why Marvin was convinced he could make her come back, why Anna is telling Michael that he could potentially see Lina...

A problem I can see is that, if there's no windmill in the first place, why is Paul supposedly getting coordinates of where the windmill was? Well... It could quite simply be where the accident happened in 1977. And since it's been established that the windmill is gone, Paul wasn't surprised not to find much when he physically travelled to the place and found nothing.

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Tiara, Belle and the rebirthing process

But then... What does rebirthing actually do? Well, it could be switching two people from two different states. Well... At least that's what Marvin and the gang actually think it does, but really, with the "duplicity theory" going on, what tangible proof of a successful rebirthing do we have?

Who knows when this idea came into Marvin's mind, but I'd personally say it was quite early on. And that the had actually some proof of it (or Marvin has just lost his mind from that obsession with Lina and he just did whatever he thought was a good idea). I think Marvin had the time to try it with a bunch of kids (reportedly) before trying it on Care, and he thought it would really work on her because of her lack of eyebrows. But I personally think he tried another kind of rebirthing before really focusing on Lina.

One thing that's been upsetting me for a while is the whole thing with Tiara/Belle. Who are they, and what happened to them? As usual, in Petscop, there are very little clear evidence of anything but here's how I think it could have gone. There were two girls named Tiara in the family : first a Tiara who would have been Marvin's mother/sister/cousin/other female relative who would have had knowledge on rebirthing and somehow disappeared/died.

Why would that be? Because there are two instances of someone referring to something Tiara said or did. The first one is "Tiara says young people can be psychologically damaged “beyond rebirthing” " on a note during Petscop 3 and "Tiara Plays Bad Music Too" said by Marvin in the end of Petscop 11. Now, we also know that Belle was supposed to become Tiara, but quit halfway through, so it wouldn't make sense for Marvin and Rainer to say these things about her. And since Belle was just a kid, it's highly improbable that she'd even say that first quote. That's why I think Tiara First of Her Name was someone older, that was in on the rebirthing schemes (theory n°5 has an amazing plot in which Tiara was Marvin's manipulative mother, you should go read that as well).

The second Tiara would have been... Anna and Marvin's first child (theory 4). Okay, this sounds like a stretch. But hear me out : if Paul truly is the half-brother to Daniel and Michael (theory 6), then why doesn't he remember Michael when seeing his grave in-game? (could be because he literally was 3 years old when Michael died, but I'm going to add something to this) You know how Paul and Care are the same people across two realities, but their genders and names differ? Could it also be the case for Michael? Michael is from Care's dimension, and his counterpart in Paul's dimension is a Tiara Leskowitz. So this means that Tiara Leskowitz probably also died in 1995, like Michael. Which would also explain why that doesn't raise Paul's eyebrow, since, again, he was 3 when she died. This is an olympic type of stretch, but hey that's why we're here for. But this has the merit of explaining a few things. Why Anna is so protective of Care. Why Anna is associated with the color black : she is mourning the death of her daughter. And when reading the "Your New Life Letter" addressed to Tiara Leskowitz, why Anna writing to reborn Tiara sounds so overly loving and possessive.

Then there is a little detail that makes perfect sense. The egg we find in the locker in the school. Pink and purple. The colors of respectively Michael and Tiara. Just like the egg we get after we put Care B in the machine is red and yellow, respectively for Paul and Care. Two different versions of the same person. The only thing that upsets me here is that Michael would have been Anna and Marvin's son when I thought he had to be their nephew. But it could be explained by the family trees being slightly different across dimensions, but I really don't know.

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Okay so... Back to rebirthing. Before trying to rebirth Care into Lina, Marvin could have wanted to rebirth Tiara Leskowitz. I have the personal theory that the scene where Paul as the SMM gets run over by Marvin's orange car in P22 is actually showing how Mike died, being ran over by Marvin's car (with the theory that Mike is Toneth). So what happened in both Paul and Care's state of things, is that Marvin accidentally killed Tiara, his daughter, with his car. That happened in 1995 or before, since in Care's dimension, Michael had the time to play a lot of the game before he died. So, out of remorse, he decides to try to rebirth his daughter and, thanks to the game, Marvin selects a girl he could try rebirthing late-Tiara into. That was Belle, the only kid able to finish all the puzzles of the game. Belle famously quits halfway through and never became Tiara, instead producing an egg. In theory 2, we have reasons to think that an egg is always the outcome of a failed rebirth.

Now, we also know that Rainer was more or less involved in Belle's rebirthing and that, more importantly, his game had an important part of it. But why would he agree to that? Well, I think Marvin could have somehow learned that Tiara L. and Mike were linked, and that by bringing the Tiara from Paul's dimension here, it would be a way to also bring Mike back. But hey that's just a theory I have for all of this mess to make sense.

But then... What happened with Belle? After the failed rebirth, instead of having a brand new Tiara, there is Belle, an egg and the note in the locker that Tiara will never read because she never was "created". So she never becomes family (as in; not a Leskowitz (through Tiara the second) nor a Hammond (through Mike)). But in Paul's reality, she still is upset that Paul says she is not family. But if she was just a random kid and a friend of Paul's, why would she expect anything different? I have two theories.

• The "reasonable" theory is that, in this dimension, she grew up to become Paul's fiancée or something. In that case I understand she would be upset hearing that from her fiancé.
• The "full duplicity" theory is that the rebirthing didn't fail 100%. Belle didn't really become Tiara, the two girls did not exchange dimension but Belle was "duplicated" across states, into Paul's. So there are two versions of Belle : the one in Care's state thinking and insisting she became Tiara ; and the one in Paul's, upset she still isn't considered family even though the transfer kinda happened. That would also explain why Belle appears in the game but cannot be on the phone with Paul at the same time, and how she knows so much about Petscop.
  

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Conclusion

The duplicity theory, as far-fetched as it could seem, explains a shit-ton of things.

This whole thing was long. If you read all of it, congratulations because holy hell this was a lot.

I'm pretty sure there are plotholes all over the place, and if you find some, please tell me so we can try to figure this thing out with what we have. While writing this I even confused myself with some details, so some things probably don't make sense. "I almost got lost myself..."

And if you have other pieces of evidence confirming what I say, please don't hesitate!

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In any case, good night.

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(PS: I hope it was okay tagging you people in the beginning, sorry if it wasn't)