PRoblems with Chico

Up until a few days ago, Chico has been the perfect puppy. He is one hundred percent house broken and he sits and shakes on command. He is not agressive whatsoever.

Well the past few days he has been growling and trying to bite my nine year old sister. This is very odd because she is one of his favorite people. WE walk together to get her from school at least twice a week and his tail just wags a mile a minute as soon as he sees her. He doesn't do it all the time though. Sometimes he is perfectly happy and playful with her, then other times he isn't . And she doesn't do anything to provoke him, nor has she in the past. So I do not understand what is going on..Please, any suggestions would be greatly appreciated.
~Jamie

Oh, by the way...he does not show this behavior toward anyone else that I know of yet

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.

Chico is three months old. Thank you a lot for your suggestions. We will definitely be trying that!
~Jamie