I'm sorry to hear that, but if you have any chance of living your own life… stay away from "what if"s, they'll eat you alive and you can do nothing about them.

As others have said, if you want to do something about your niece, go ahead and do it the legal way. Still be prepared for a shit storm if you go that way; I would suggest trying to do something anonymously in a way that your brother doesn't find out it was you, but if your niece's safety worries you, and you yourself have a safety net to fall back onto, then by all means go all in.