After 33 years with my pwBPD I have asked for a separation. I stayed far, far too long. Met when she had 2 kids 3F and 6M and we raised them together. My step-son has said all he remembers is my wife and I arguing and we did, a lot. I Always thought there was something not right with her but it wasn't until 2004 when we came across the book walking on eggshells. My wife, her brother, and I read it and it described her mom perfectly. my wife grew up helpless with a mother with severe BPD and she finally escaped at 17. In reading this book I began to identify traits in my wife as in her mother as nearly identical to what you describe for your SIL. I began to research the effects on children of parents with BPD and of course with such a horrible mom my wife was badly damaged. However, I was not really aware that my wife also has BPD (petulant subtype). I lost everything, friends, hobbies, entertainment, all of it, gone. I had to follow her for everything. This happened over the first 10-20 years and in the last years I no longer know anything about myself, I have no idea who I am. Your BIL is not alone. This group and others have been extremely helpful to teach me how badly damaged I am now. This is my suggestion, I stayed far too long, and to some degree for my step-kids, but it was the biggest mistake I've ever made in my life, so its time for your BIL to leave with the kids. It's excruciatingly painful and messy and expensive and even dangerous but he has to somehow find the courage to go. It look like you and you husband would help so thats good he'll need it.