I'd like to add my thoughts on this...
First, if you're fostering this cat, isn't the decision the responsibilty of the agency you're fostering her for?
That aside, I would really recommend you see if you can reinstate and keep that appointment.
First, you've been told by a vet that's she's not pregnant. You have to go with that.
Second, you're not saving anyone. If for some unforseeable reason she is pregnant, and it's in such an early stage that it's undetectable, it's not like doing it when they'rfe 5, 6, or more weeks pregnant.
I also dont' believe that we, as humans, should put our values on animals. It is a very different situation. In other areas we don't. We will euthanize a sick cat or dog, but we will not do that to a human, and we understand the morality of stopping an animal's suffering.
It's the same here.
You may see it as saving lives. Say she has 4 kittens, and you let them grow to term, and then you get them homes. You think you've done the right thing.
What you're not seeing are the 4 kittens that might have gotten those 4 homes, and didn't because your cat added 4 more to the world, and the other 4 and now left alone abandoned on some street, starved, dying from flea, tic and god-knows-what infestation, beaten by kids, wounded by other animals and left bleeding and to suffer for days on some street somewhere until they mercifully die, crying, starving, alone.
Because I believe for ever kitten that born into a luckier situation, that leaves out one who desperately needs to be saved.
So which is the moral action?
I had to make that decison about my Deja Vu when I found her. The reason I did not have her spayed while pregnant was because she was already 8 weeks pregnant. At that stage I would not do it, it's not good for the mom either.
But if it had been just a week or so, as sad as it would have made me, I would have done it. Then I would have gone to the rescue group and asked to foster a mom who needed a place to have her kittens because she was almost near term
I still think the vet is right, but the longer you wait, the worse it would be if she was.
Sadly, at least for me, it comes down to a decision of am I going to do what makes me feel better, or am I going to do what's best for the cat and the babies, and any other babies it might affect.
I would have a harder time living with a decison that made me feel I acted selfishly by doing what made me feel good. I could live easier with a decision knowing that decisoin not only was the best all around, but it saved some other animals that would have ended up suffering.
Again, this kind of decison for a cat (or a dog for that matter) is a very different issue than it would be for an human.
Your cousin was right, and yes, has probably seen the horrors out there, and wants to save some more babies from having to go through that.
Think about it. Wish I had seen this post earlier.