My brother in law is a serial cheater. He and his wife have been together since 14, together for around 10 years. Since high school onward, he’s always cheated on her. In the times she’s found out, she’s been crushed/angry but stays. At one point, he thought he got one pregnant. She didn’t find out about that.

My BIL is an asssshole. When my bf and I split the same chores, he asked me if I thought I was “the man of the house.” He’s straight up told me he hates women, thinks they’re useless & dumb. When he’d work away from home during the week & come home on the weekend, he’d flip and scream at my SIL if the dishes weren’t done, counters cleaned, carpet vacuumed, laundry done, dinner made, etc.

She is a procrastinator and a doormat, and I knew if the cleaning wasn’t done, he’d destroy the whole house’s weekend like he always did. So every Friday after school and work, it was a shit-show of me cleaning up all of her dishes, making her bed, folding his laundry, etc, to cover for her and preserve the peace. He thinks she should be a housewife or Hooters Girl, no in between. He threatened to burn the house down one time because a male coworker wished her a happy birthday. His wife has to make dinner, make his plate, and *hand feed him*. Anytime he screams at or mistreats her she does nothing, or at times, defends him. She is extremely dependent on him. Overall it’s toxic. So much more, but this is getting lengthy.

This summer, he cheated once again. The girl is 19 and in college. It was a secret up until the fall, when bf and I figured it out. After watching a lot of Sister Wives, BIL said he couldn’t imagine 5 wives, but “2 or 3? I could see that.” After a lot of convincing and manipulation, his wife decided it wouldn’t be too bad, but neither were serious. Talk of threesomes, eventually she says, “maybe I’d try it someday” (she’s hetero) and he said “I might know someone who would maybe want to try, that we could trust.” Cue him running to side piece and bringing her into their intimate space. His wife put herself out there, made herself vulnerable, nervous to try but willing to explore with her “trustworthy partner.” She has a threesome with her husband and, unbeknownst to her, his mistress.

So she thinks this is organic, just a girl, and they all hit it off. He convinces her to let the girl stay a few nights a week even tho she didn’t enjoy the threeway, and it has turned into a sister-wife arrangement. He splits nights with them, they clean the house, he brings them around my bf, whole 9. This whole time, SIL doesn’t know the truth about this girl. Since then, SIL has become pregnant.

I’ve been away for school, but BF says it’s clear BIL favors side piece when they’re all hanging out. SIL also looks extremely jealous. I think she’s letting this happen because she knows he’ll cheat, and maybe feels she has “control” if she knows who he is fucking? IDK

I’m not supposed to tell her because it would ruin BF/BIL’s relationship. BF is extremely conflicted and wants to tell her, feels awful, and a lot of guilt. However, he feels in debt and loyal to his brother. I do not. In fact, when I move back with him this summer, I don’t know if I can even see SIL. I am so angry and want to tell her, but it’ll cause chaos between BF/BIL, and cause friction and mistrust between us. But I cannot handle this. She is about to have a kid, the side piece is barely an adult, and my BIL is living a lie. This whole thing is a mess and I feel complicit. I cannot hang with them like a family with this elephant without blurting out the truth.