Niece is mean to my son

I love my niece (20 months). She is adorable and I don’t think that she is mean but definitely lacks discipline. She snatches from, hits, scratches, and bites my son (15 months) repeatedly. He had been left bleeding more than once and with a bruise on almost every encounter! He simply takes it and cries. I don’t want him to hit back of course and on the rare occasion that he does hit (usually me or his dad) he is sternly told “no” and put in a momentary time out. Her mom does absolutely nothing. She apologizes afterwards but does nothing to stop or discipline her. It has gotten to the point that my son is scared of her. It makes me so angry (not at my niece but at my sister). I don’t want to discipline her because it’s not my place but at this rate we don’t really want them around anymore. What can I do? via /r/Parenting http://ift.tt/2Gh5NLS