No longer friends!

[Alicenwonderland]

I have two cats, a boy and a girl who are now just over one, we got them 8 weeks apart. They've always got on okay. I've just had the boy neutered and have been letting them both out. Since then the female has become very aggressive to the male whenever he comes back in, hissing and trying to attack him. She hasn't been neutered yet as she has shown no signs of being in season. Has anyone had this before?

There are too many unwanted kittens, it's irresponsible to breed your cat just for the entertainment of watching your cat have kittens, and then flog them off to new homes.

Hoping she’s booked in to be spayed now. Then she can go out and won’t feel the need to attack him when he comes back from where she can’t go! Welfare first.

OP is already letting her out

Why don't you have a Google at feline mammary cancer and that might make you have 2nd thoughts on neutering her.

The most ethical and responsible way to experience kittens being born is to foster a pregnant mum cat from a rescue. Could you do that OP? As you have a big house and are well resourced you can probably spare a room to house mumcat and kittens.