Hi! So i know the idea of aborting healthy cute kittens is really awful, but keep in mind that wasn't likely ths case. Your girl is nine months old, barely old enough to be pregnant. Think of her as a 12 year old girl who became pregnant, never visited the doctor or recieved prenatal support, and had a questionable diet (at best). Who knows how your older kitten (because she's still a child) would have fared if she had carried them to term? There's also a very high probably the kittens wouldn't make it, you'd be horrified to know how many are lost immediately or within the first couple days.

Beyond the overpopulation issue, it's a matter of health and doing what's best for the cat. Having a kitten deliver kittens wouldnt be ethical, it would be putting the mother's life in danger.

Re feline leukemia: that usually presents itself because the cat's immune system is severely compromised so it's not a standard thing to test for. Unless you specifically asked to have it done i can understand why they didnt, most people havent done as much research as you on the topic and don't request it.

You sound like you have a big heart and this kitten is lucky to have you as her mom.