Just seeking advice.

Bare with me as this will be long.

So I have a sister who I am or well was very close with. She has 3 kids. 1 in particular from another guy who she doesn't know when she was 16. She's now been married to a guy for 10 years and that's where the other 2 kids come from. Well her husband is a POS. He's always been verbally abusive to my sister, sometimes getting physical with her such as pushing her or grabbing her arm. But more so verbal. As of lately within the past year or 2 I've really started seeing his true colors and how manipulative he is. I wish I really could go into detail about this guy but it would take such a long time. He's just a shitty guy, I mean truly. Lately it's been ridiculously bad. He verbally abuses my niece the one who isn't his. He makes her do everything around the house while his blood kids do nothing but act like entitled brats. He's told my sister how he doesn't want her or my niece looking for her real dad at all because he's her "dad". However he's nothing like a father figure to her. He can't support her such as buying school supplies, clothes, or pay for her sports and so much more. He can barely even pay bills. He makes my sister pay all the bills. Plus lets not forget he verbally abuses her all the time. Something a real father wouldn't do. He got mad at her one time and caused a huge fight with my sister just because my niece had got seamless underwear to wear with her spandex for volleyball. Ridiculous right? Freaking seamless underwear!! Not like a g string or anything. Well so anyways lately my sister has been talking about how she's unhappy with him and feels nothing for him anymore and she's ready to leave him. Well last weekend they just happened to get into a huge fight because she simply took her phone into the bathroom with her while she took a shower. The phone she pays for. A phone that doesn't even have his name on the plan. Which of course he had been drinking also so the rage he gets with her is worst. Well he gets mad because she had texts from a guy from awhile ago about getting a test done to see if he is the dad to my niece. Which yes was wrong for my sister to hide and lie about. But so he gets so mad he pushes her, then starts throwing all his clothes at her while calling her a slut and saying all these other things. The kids were crying and scared during all this (her words) so she tells me again how she's just so done and blah blah. Well she ended up asking me if my husband was home because hers took off to his moms house. So If he were to come back she was going to call me so that way if anything my husband could beat his ass. Well the next morning comes and I text her asking if he came and she said yes. So she let him come back. So I ended up texting my parents about it because I've just had it. I've had her letting him abuse her like that and she allows it and my parents never know about anything like this happening and when they do she always covers it up. So of course I tell them and that's exactly what happened. She told my parents it's all her fault, and that I blew it out of proportion. So of course my parents just sweep it under the rug like always and my dad has the nerves to give her husband the benefit of the doubt about it all. So of course because I can't stand the POS and see right through him I'm the bad guy. So I am pretty much done with my sister. I want nothing to do with her. I'm at that point where I'm not even going to allow her to come see my son when I have him next month. The only time she will see him is when there is a family event. That's how I feel anyways since she's being so blinded by this guy it's just sad. What would you do in my shoes?