can you help me figure out what's causing my cats to fight?

Yes, this could have caused it. It's referred aggression where the cat can't attack the object that scares it so it attacks the next best thing. I think you need to have the boy neutered and maybe separate them for a short time to let them cool off a bit.

Since the boy is not neutered, he may be trying to mate with his sister, even though she has been fixed and is not in heat. In this case the male wants to mate even though the female doesn't. (They don't care if it's a close relative.) Male cats that are not neutered can be really aggressive, and I have seen them even try to mate with other males. When they are getting ready to mate, they act like they are fighting. The male will come up and bite the female on the back of the neck.

If you get your boy fixed, he will most likely stop doing this.

If your male cat's not neutered, that's your problem. Male cats tend to have territorial issues and take it out on other cats. I work at an animal shelter and whenever a cat that hasn't been neutered comes in, it always picks fights. Its a guy thing.

There is some sort of stress -- change -- going on that is causing them to attack each other. Absolutely have the boy neutered; he'll mellow right out.

Are they screeching when the fight, like meow and cry at the same time? If they aren't then they are just playing. If they are take them to the vet and ask them why, don't even take them in, just call them.

hes not neutered so he will still try making his sister angry