If the introduction is done properly, especially with a kitten, they should get along. If there's a question about who goes to the Humane Society, it SHOULD be the kitten, as it will have a greater chance of getting adopted. Please beg your aunt to try and introduce them the right way. If it does not work out (which it sometimes won't), explain to your aunt that a kitten has a better thance of finding a furrever home as opposed to an 11 year old cat. Besides, the older cat was there first.