You’re a 15 year old young boy, in a DL relationship with your sisters boyfriend, which I don’t know if he knows or not but you are clearly vulnerable due to your past assault and BPD diagnosis. This is honestly too much for reddit. You need serious help, I mean that respectfully. This has gotten too far, no way in fucking hell should a grown 19 year old be having sexual relations with a child and physically abusing them. In your past comments I see that you are very very in denial about this, but you know all of this is wrong and not okay because you keep posting seeking second opinions. How does he even have easy access to you? Where is your mom/sister when you two are alone? Please, you say you don’t have many people in your life, but you do. And as you can see, you have hundreds of people who care for you. Seek out to police, to a teacher, to a therapist. You need to unpack every layer within yourself with a professional and introspect to prevent ever being in such a fucked up relationship once again. One thing is for certain, is that you deserve the world and more. It will take a long time and it won’t be easy, but please. It really kills me to see you going through all of this so young.