I hope she recovers really soon!

I agree with others that you should definitely confront your sister and tell her your concerns. I find it very suspicious that she sustained a spiral fracture from just sitting on the floor and not being on the move... I'm sorry and I hope I don't offend, but from what you've said about your nephew, his past behaviour, the fact that he lied about the situation, this would be the last straw for me and he would not be welcome around my child until his mother acted upon my concerns and I saw improvement in his behaviour.

Your sister sounds as though she knows her son is in the wrong as she is acting very defencively, not questioning her children, blaming you and not taking one bit of responsibility. Appalling!

Good luck and please speak to your sister and don't let your nephew go near your daughter without YOU there, because unfortunately, it doesn't seem like you can even trust your sister to own up if her children do do something wrong.

xox