Huge feud between sister, sister in law, and myself. How can it be repaired? What to do next?

Anonymous
This all started when out of the blue my sister in law of ten years made a Facebook post about how she basically considers my brother’s immediate family (his siblings, parents... etc) deadbeats in her kid’s life. She claims we don’t even talk to or check in on them. The truth is this woman is a 28 yr old with three kids that parties like a 16 yr old and post trashy twerk videos online. She can’t string together the most basic intestinal conversion either. I always babysit her young kids and my parents always pay her rent because she’s always getting fired and evicted. Needless to say she lied. I made a generalized post the next day and she thought it was about her. Also, my sister made a post directed to her about her lies. Anyway, they get into a huge argument in my comment thread (over 500 comments) arguing. In that argument it is revealed my sister in law is even more disgusting then we thought airing out vulgar details of her sex life as a rebuttal. She’s also apparently racist as racial slurs started to come out towards my sister. They’ve threatened each other as well. This is all coming from a woman that never had a problem with us before in a decade of marriage to my brother. Anyway, the things she said about my family (lies and opinions) for the public (I have no room to type them) make it almost impossible to come back from. This was all really bad timing as a family member of ours was murdered earlier this week in a homicide-suicide and are still emotionally healing. On top of that the majority of my mothers family who are all also mean trashy lowlifes (minus my immediate family) are close to her and are looking at my sister and I like the bad guys when we’ve just been defending ourselves. Even though I’m not at fault I just want there to be peace but I don’t know how.
Huge feud between sister, sister in law, and myself. How can it be repaired? What to do next?
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