I just wanted to give an update on a situation I had asked for advice about here. It's been nearly a year since I posted and I'm sure folks won't remember (the post is
here), but I never forgot your thoughtful and supportive responses during a time when I was feeling pretty flustered with the whole situation. A very short summary: I was feeling pressured to babysit my niece who was having some behavior issues due to a difficult home life and wasn't sure how to respond to my SIL. I'm happy to report that things have gone really well since then.
My SIL divorced in the spring and while her abusive ex is still in the picture for shared custody of my little niece, he's been mostly behaving himself. My husband and I stayed consistent with turning down requests to babysit and explained briefly we weren't comfortable with it- she respected that. We help in a bunch of other ways and regularly spend time together as a family. I took the suggestion of recommending SIL find a sitter through caregiving websites and she's currently using one to find somebody- my niece has a list of qualities her new babysitter should have, a la Mary Poppins.

We're also looking forward to spending a week in Florida with them this spring for her first trip to Disney World.
Anyway, I wanted to update because it's easy to think when you give advice online that it doesn't matter or nobody is listening. Well, I listened and I really appreciate the 3FC posters who took the time to respond so kindly. Thank you!