Very good for you to think of this! Yes, it is possible for Parvo to be tracked into a home or yard from a public setting (by a vaccinated dog) and passed to an unvaccinated dog. However, this doesn't necessarily mean you must limit your dog's activities. Your brother can limit his puppy's exposure to Parvo and other illnesses by keeping her confined to an area where only she will be. In other words, until she is vaccinated, your dogs should be kept quarantined. Each should have their own places to potty, eat, sleep and play. And, unfortunately, contact between the two should be kept at a minimum.