I think you are following you heart and your right to your instints and opinions as a human being. I do not think you are being selfish at all! Their feelings are hurt, so they are lashing out at you because they didn't expect this response from you and your feelings are hurt because you are shocked that a family memeber purchased a puppy from a pet store. Perhaps let things calm down for a day or so and explain to your mom what the risk or parvo is to your pups and that you are going to stick to your guns on this one but that you are not trying to hurt anyone's feelings. I would also inform your family member of the parvo outbreak from the puppy store....
Good luck!