We can choose our friends, but relatives are thrust upon us. I have a few who I would not choose as friends, and a couple who I am greatly blessed by NOT having had contact for nigh on 40 years.....may it continue thus.
Now you know that your SIL is as bad as MIL, and if they are in contact with STBX it puts them in his camp, one you want out of. She wants to be pally with folks like this, then she is not the friend for you.
As for some of the things you did to STBX, well....he should be grateful that tools and mobile phones got it, and he didn't. Obviously he had pushed you a bit further than was safe, so be it......now he will be free to play his game with someone else and you will be free of him and his games.
Let SIL, MIL and their FB bitch session fade into nothingness, and go pamper yourself for being who you are....someone who doesn't need to bitch to feel ok.
God bless