poor little poppet. I don't know anything about how breaks happen but I do know they heal very quickly. I hope she gets better really soon.

As for your sister and her children, I would suggest sitting down with her and expressing your concerns and see how it goes. Hopefully she is aware of their temprement and understands why you are so worried. She may be blaming you as she doesn't want to admit how naughty her own son is. Alternatively don't let your lo play alone with her children. always keep them supervised and watch their behaviours very carefully.

HTH