I ride Patchy most of the time, but my little sister also rides him often. EVERYTIME she rides patchy has learned he can pull the reins out of her hands. He bends his head real far forward and it even brings her forward in the saddle. He doesn't do this to me because I get aggressive with him when he does, (he pulls, I pull hard back and quick right after the action.) It's gotten to where he tries that with anyone that even gets on him! I can't fix the problem, when i'm not the one riding him. What should I do?