PLEASE READ & HELP: I've been having sex with my brotherinlaw for about two years, should I tell my sister? If I do, in person or through text?

Anonymous
I started living with my sister, her husband, and their child. I'd been staying with them for more than a week when we all fell asleep in the living room while watching a movie. I woke up to him grabbing my breast, I was in shock, I had always thought of him as an older brother. He and my sister had been together for years, he had watched me grow since I was 9. He saw me go from a fat little ball to a decentlooking girl. I grabbed a hold of his hand, but I didn't take it away. My self-esteem was low, I had recently tried to commit suicide and was vulnerable-- these are the lame excuses I am thinking of telling my sister if I ever do.
Later in the week, we ended up having sex. Somehow, I started smoking cigarettes as he did. Somehow, I started skipping school to have sex with him. Somehow, my sister didn't suspect a thing.
I feel bad for doing his to my sister, I've told him that I feel bad multiple times, and when I tell him about ending whatever we have, he gets mad and manages to create a distance between me and my sister and nephew. Soon, I start to fall back into his trap. As soon as I give him what he wants, my relationship with my sister and nephew gets better.
I've managed to get away with only "teasing" him and not have to have sex for some weeks now. But I'm tired, I feel filthy, and there are more things in play now. My sister wants to have another child with him and wants to give him papers. I think he came onto my other sister before, but he told me it was her that came onto him. I don't know if I should tell my sister about this, I don't know if I should just continue to keep it all a secret. All I know is that I'm not going to sleep with him, I don't need my sister's husband to like me or tell me I'm pretty anymore.
If I tell her, I'll break up her family and she may never talk to me the same. If I don't, she'll have another child with him, she'll continue the residency process, and he'll keep on trying to have sex with me.
What should I do?
PLEASE READ & HELP: I've been having sex with my brotherinlaw for about two years, should I tell my sister? If I do, in person or through text?
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