My 3 month old lab puppy will run after the ball when my older lab (who has had hip sugery) is playing fetch. She thinks they are playing together and will grab the ball from her.I can keep them separated when we are at home, but as they grow older and they travel with us or go to a dog park, i won't be able to do this. Is there any way to train her to know when the ball is for her and when it is for her sister?