How old is Chico? Lots of puppies go through a stage where they are suddenly fearful of familiar things, kinda like a baby making strange. Unless he is really threatening your sister, I'd have your sister only do good things with him. She feeds him, plays with him ect. All good things come from her. She can't do any of the disiplining, raise her voice ect. Only fun stuff. Again, don't put her in a situation where she could be hurt, someone older should be supervising. All fun stuff stops when he acts inappropriately. He gets a time out in his crate, she turns her back and ignores him--whatever works for your puppy. My dogs hate a turned back, drives them nuts and they soon learn to not to do whatever made me turn away.