Yikes. Sounds like things are falling apart over there with your W and your poor kids are having to deal with the consequences. I'm not so sure that it's "not your position" to say something if it's truly affecting the health of your children though. I'm not experienced in dealing with sitches with kids, but if those were my kids, walking around, smelly, dingy, greasy, bad teeth, and lice, I'd be calling my L, yesterday.
Also, if your SIL is truly concerned about the kids, and she continues to not speak up to your W for whatever reasons, but instead just keeps telling you about everything... Maybe should could help with the "documentation process" by sending some pics of what might be going on over there (the fridge/food/alcohol, living environment, etc). I wouldn't tell her that's what she's doing (providing documentation), but asking her to send pics of things so you better understand what it's like over there... isn't going to hurt anything.
