How should this situation be handled? Long post with back story and all

I am tearing myself apart trying to decide how to handle a confusing and difficult situation. So here's the story/situation.

My sibling is in a kind of new relationship two years give or take a few months. Right from the get go my siblings significant other treated my sibling really terribly. Like telling them what they can or can not do what they and their (siblings) children can or cannot wear and who they could be friends with. I was thinking new couple, basic insecurities, first year jitters.. however that escalated from (semi) innocent suggestions to full blown controlling behavior. Naturally I called out siblings s/o (yes I know stupid move). They went on the down low. Then the s/o started insulting and spreading rumors about me. I confronted s/o again so here I was looking crazy because oh s/o is so "nice". Sibling was not even questioning how s/o treating them and also decided to talk about me. Ruined my relationship with sibling for a time.

Here is the reason behind post: come several months after fall out with sibling she comes out with you are right now they are calling me names controlling my friends life and hitting me. I helped them get a way out. They then decided nah I'm going to stay. I was acting crazy and wanted "attention" they aren't actually abusive. And again a few months later same shit. Controlling abusive and mean are the complaints. few days later same shit I was just angry they aren't abusive. I told them they are fucking stupid if the stay in an abusive relationship. so now both sibling and s/o are talking about me again saying I'm saying shit i'm not. I can't be around them anymore. I really want to beat on their S/o so they Know what it's like. they are both trying to talk to me I want nothing to do with their relationship they somehow keep dragging me into. I've been avoiding them like the plague but I notice the pattern it's not going to go well when they realize I'm avoiding them. s/o going to get my sibling worked up playing their anxiety like a violin get them mad at me and once again spreading lies that cause other people to think I'm mean and close minded and my favorite... "jealous" cuz oh yeah they got it so good and I really want to be treated like a door mat😉(petty I know but so tired of that getting back to me) joke aside what should I do to keep out of this toxic relationship?