Paul is Michael and Daniel's (Rainer's) younger half-brother.

They have the same mother, Jill Leskowitz.

Roneth is a representation of Paul himself.

Now to answer some questions on this.

Why would he call Jill by her name?

It's not uncommon for bad relationships to develop with a hostility but it would explain why he is living with or is around Jill. The conversation we see between them play out in the game is very hostile indeed. He may actually call her mother when he is talking to his now confirmed friend and tells them his mother had the game before him. Easily could be referring to Jill because Daniel made the game, of course she would have it after both her sons have passed away.

Why doesn't Paul remember his brothers?

Michael would of died when Paul was 3 and was pretty young himself. Rainer may have left home after Michael died and his obsession with Marvin began. Paul remembers Rainer at Michael's birthday party taking the children from Michael's school into the basement to test the game Petscop. Now: why would Paul be invited to a Birthday Party at the age of 3 for a kid several years older than him and why would he remember not being with the other kids or remember this game? Because he lived there already. Care did play-test the game and is a cousin so her being there makes sense.

Roneth is Paul?

Yes, Roneth is described as Toneth's younger half-brother who looks up to him and always remember to look both ways before crossing the road. Toneth is depicted with limbs broken and a description that heavily infer that he was hit and killed by a car but it also connected via the Painting Puzzle as being a representation of Michael. If Michael had a younger half-brother it could not be Daniel. We know Daniel is the older brother from the end of the Petscop Kids credit roll.

Why would Jill be a Leskowitz?

We know Paul's last name is Leskowitz because of the reddit post. We know Care is the cousin of Michael and Daniel. Michael and Daniel have the last name Hammond. If Paul is there half-brother then he would not have the Hammond name but with the other father (could be Marvin even) out the way (or hushed up, we know Anna and Marvin have marital issues for some reason) Paul assumes the Leskowitz name. Anna is Care's mother and is a Leskowitz, Marvin is a Mark. Which means Jill is either a Leskowitz or a Mark herself. We have just discovered Tiara is a Leskowitz, the idea that it's four sisters (Anna, Lina, Jill and Tiara) is not a far-fetched idea.

This would explain why Paul is part of the family and connected so intricately with it. It's also why he's referred to as Newmaker at one point, the same as Rainer signs off his letter with. They are the Newmakers.

Essentially the tragedy of his brother Michael's death and the suicide of his brother Rainer and the kidnapping of his cousin Care and his aunty Lina's disappearance were all dark family secrets covered up and kept from Paul growing up.