Well… I have to agree with your vet. Although you have to remember it is double the trouble, expense and training. Although doubling the pleasure is BEST! Instead of a sister, I would consider a brother. Same sex sometimes has difficulty later on, jealousy etc. Be sure they are spayed and/or neutered. At first it might be difficult because they would both be under foot but as time goes on they will be entertaining themselves elsewhere in the home. There is nothing better and more entertaining than two of them doing “zoomies” around the house. I guarantee that will be a delight to watch. It might take some of the closeness away from you and her at first, but actually that would be for the best. You don’t want her to become so dependant on you that she will become neurotic and have separation anxiety anytime you leave her.