That's great how you decided on wanting to add a beloved cat to your family.
Personally I cannot see how your family thinks your are hoarding animals, but that is their opinion.

One question I have for you. Is your sister within walking distance? Why I ask this, is because I was thinking that it might be good if you could bring your dog over to your sister's and introduce her to one of the kittens to see how she would react to it. Then continue to do this on a regular basis and start having them slowly interact with one another. This way your dog would get use to having a cat around.

If you do land up getting a kitten or cat and want it to go outside, please train it to wear a harness and leash. This way you know it won't go astray and land up having something happen to it.

Good luck.