The dog my mom and I helped save had bit twice, both times while she was pregnant. Animal Control ordered the dog to be removed from the city limits. My parents live out in the country and agreed to take her. They (my parents) believed she was only "aggressive" because she was being protective of her puppies. Although we knew she was not good around small children. That wasn't a problem because my nieces are 13 & 16 years old. However, one day my youngest niece took her little 2 year old cousin over and let Pepsi out of the kennel. I'm not sure how it happened, but Pepsi bit him. It was completely my nieces fault for letting her out around him.
But anyway, enough of my stories. I hope things work out okay for your sister.