My cat, Mimi, is now eleven years old. When she was 7, we tried to introduce an abandoned three week old into the house. She hated him even with him being so small. As he started growing and got healthier, she got worse. We couldn't get near her. She had always been a very loving cat, but soon she wouldn't come inside, wouldn't eat, etc. When "Dwayne", my daughter's name for the baby, went off for the weekend with my brother, who was living with us at the time, Mimi would just brighten up and act like he had never been there. We finally had to make the decision to find a home for him and were successful in doing so. He is now a huge, beautiful gray cat and lives happily with the people who adopted him. We will be a one cat family as long as Mimi is with us. We have a "neighborhood" cat who comes to eat twice a day now and she can't stand him either. At least he isn't in our house. She has learned to go out the back door rather than risk running into "Thomas" on the front porch.
I wish you luck. My sister got littermates (2 females) and says that is the way to go. They entertain each other and get along beautifully.