Financially, your SIL is probably far worse off, and the majority of women who divorce (and their children) end up living in near poverty, financially, than divorced men who do not have custody. (I must try and find that stat.)
However, in your SIL's case, where there has been so much abuse, I think she is better leaving the M. Once she gets back on her feet, financially, then things will start looking up for her. She will need her family and friend's support right now to uphold her decision (I went back to my abusive first husband because I had little choice ... my family and friends would not or could not help me or support my decision. I left him as soon as I had a job, and could pay for the D, a year later ... best thing I ever did for myself, and I learned that the only one I can truly depend on, is ME). Good for you for having the kids stay over for the summer. It will give SIL a much needed breather from the guilt of not spending enough time with them.
Abuse and drug/alcohol/porn/gambling addictions are the only reasons I will ever encourage anyone to seek a D. Adultery (especially if it's only once) can be overcome with understanding, compassion, hard work, and forgiveness.
Well, that's muh 2c worth. Glad you had fun last night, and good that you vent here, and not at home.
Me:57 H:52 M:28 Got another lawyer last year and filed. D35,S/D twins28,D22 EA4/04 End? Who knows? "Life is like a mirror. Smile at it and it smiles back at you." — Peace Pilgrim