Take a big step back. Take a deep breath. And look at the whole mess in front of you.
Your W and BIL have created a dysfunctional, destructive situation for all involved. This is something that will have long-term consequences not just for the adults, but for the children.
It's time to extract pride, worry, fear and timidity -- and your self-interest about getting your W back -- from all your considerations. Sit down w/ your sister and talk to her. Then sit down w/ your W and talk to her. Sit down w/ your BIL and talk to him. Tell your W & BIL it must stop.
Me: 50, MLC/WW 45 Young kids Nov 2015: BD1 Apr 2016: BD2 Jan 2017: W filed Feb 2017: D final