[I'm going to bed after posting this, so sorry for any delay in response. Please include your kinks and limits when messaging me, and any changes to the setup you'd like to make, if any.]

You think you're home alone while our parents are out, and you have something to drop off in my bedroom. To your surprise, when you open the door, I'm sitting at my desk, completely zoned out. Turns out I had come home for a few days to visit before starting my new job, unaware our parents were on a work trip. You apologize for barging in, starting to make an excuse about not knowing I'm home, but you trail off when I don't even seem to notice you're there. Eventually you ask what's going on, and I explain in a dull voice that my hypnotist just finished putting me under and will be making one final review of my understanding of the triggers when he gets back. Worried about what that means, you step up to my PC and end the zoom call I'm on, sit on my bed, and begin to interrogate me. [We can do this part in detail as the start of the rp if you want, or start a bit later.]

It turns out that I've been hiding (especially from my family, of course) an extremely submissive side. I've always dreamed of being a man's slave, and now my boyfriend and I have been working with a fetish hypnotist to make it happen. It turns out that since I want it so badly, the hypnotist was able to set up an extremely powerful trigger and work with the two of us to refine exactly how we wanted it to work. I've come home to spend a last week with the family as a free woman, because next week, my boyfriend will activate the trigger, and become my Master.

You worry about my safety, but I point out that the reason I spent so long working with the hypnotist was that it's not just a simple obedience trigger. If a command my Master gives me makes me too uncomfortable, I will respond that the command is currently beyond my limits. This means I don't have to worry about establishing limits and thinking of every possible scenario ahead of time, but will instead automatically give any new idea consideration and decide whether or not to obey.

Just as you start to feel a bit better, though, you ask how long it will last, and I say it'll be permanent. You ask why I would let him set it up so there's no way out, and to your shock, I say it was the part of this I want the most and insisted on. You ask if I can be talked out of giving up such extreme control, but I say that I want it so badly, even trying would probably snap me right out of trance.

Finally, you ask what sort of safeguards are on the activation trigger. Can only your boyfriend use it? I respond that anyone can try to use it, if they know how, but it will only work if I find the person trying very attractive and trust them. It actually takes three separate trigger phrases, so that it never gets accidentally activated during casual conversation. And after the first two, the person has wait silently, without saying a word, while I decide if they meet my criteria for a Master. If they don't, I'll say no, and then forget they ever said the first two triggers. If I find them hot and trust them enough, I'll say yes, and if the next thing they say is the third phrase, they'll claim me permanently. They can still back out at that point by saying anything else.

Half hoping the whole thing is too elaborate to work, you try saying my first two trigger phrases, just to see if they'll work the way I said, expecting me to think for a moment and say no, but crossing your fingers that maybe, just maybe, it'll fail complete. To your utter shock, I instead say yes.

Of course you're not going to say the third trigger! Permanently enslaving someone is monstrous, especially your own sister! Although, it is nice to know I trust you. Maybe a bit odd that I also find you hot, but at least it's flattering. And no matter what you do, I'm going to wind up enslaved to someone, since I'm bound and determined to commit to my boyfriend next week. You certainly trust yourself to treat me right over anyone else! Plus, the way the whole thing is set up with me being able to veto anything I need to limit means it can't be too bad. If I'm really so desperate to be a permanent slave, and I said yes, I'd want you as a Master, would it really be so wrong? ...

[I think this is the most natural starting point, where you can either decide to to say something else, so that you can take more time to think about it, now that you know the trigger phrases, or you decide to take the plunge and claim me, and deal with the consequences as they arise.]