I am a long time lurker (watching Smosh, Kallmekris, The Click, etc.) and have seen some good advice come out of the comment section, so now I am posting for the first time. There is so much context I could give, but it would be very long and I prefer to get right to the point and add more later.
My sister, I'll call her Alex and her partner, Sam. They have two young girls Emma (5F) and Anna (3F). Just before Christmas all 4 of them were driving to Sam's sisters house and they saw a vehicle had crashed and spun out to the side of the freeway. Since Sam and Alex are trained to respond, they pulled over and Sam got out to help the man who was clearly distressed as he kept walking around confused, almost into traffic a few times. While my sister was sitting in the car with the girls (she was driving), she turned around to let them know why they had stopped when another car tried to swerve to avoid debris, hit black ice and steered directly into the side of their car. Anna made it out without a scratch, Emma had a concussion which they didn't find out until days later when Sam finally broke down and decided to take her to the hospital; and my sister ended up with 5 broken ribs, nerve damage in her left leg and right arm (her arm was stretched out to the passenger seat when the impact happened), and she got a severe concussion. Because of this she was not able to walk/only a few agonizing steps, and needed to basically be carried up stairs. To this day she still has to sleep in a chair because laying down is too painful (it's been just over 4 months).
Now to the issue at hand. Because I live 6 hours away from my sister, I video chat with her pretty frequently. I've always had an issue with how her partner treats her (not thoughtful, classic weaponized incompetence, and someone could call her narcissistic, the list could go on), but I bit my tongue because if Alex was happy, that's all that matters. Alex does keep things pretty secretive, but as we've gotten older and closer (I'm 29F btw), she's started to open up more. This conversation came up in February and I have never felt so angry and repulsed by her partner. Alex told me that they had one of their female coworkers over for some drinks. My sister still not being able sit anywhere but the living room chair, watched while the coworker and Sam drank quite a bit and chatted. Since the coworker drank too much, Sam offered to let her stay the night, that she would set up a cot in her office. But as soon as they got upstairs my sister could hear them making out. Most people would say should've yelled, but the girls where sleeping in their rooms and what would she say to them? I guess she could've text "what the fuck?" But I think at that point she was just done. Of course the next day when she asked, Sam denied it/didn't remember; so she asked the coworker and confirmed they had hooked up. And only then did Sam fess up.
Emma had her birthday last month and my parents, brother and I went. As well as both of Sam's sisters and their husbands. Sam doesn't know that I know, she also doesn't know that I told my brother and parents (because like fuck she gets to pretend to be a "good" person). Everyone else was able to be civil and put on a show, but I couldn't even look at her and refused to talk to her as much as possible. It's such a difficult situation because there are kids involved. My sister is making plans to leave, but she is on disability right now and it's been extended to July, she physically and financially cannot leave right now. I have told her though that if/when her partner leaves I will move there to help raise the girls (even though I have always planned on staying child-free; I would make that sacrifice). I don't know what else I can do, and I don't want her partner to refuse letting me visit for the time being. But I feel so protective of my sister and nieces, I don't know how to pretend to be civil.