2 year old female suddenly aggressive

I have 2 cats. They are almost 2. Litermates, born and raised together. They are always so loving with each other cuddling and grooming each other. Today, all of a sudden the female turned aggressive towards her brother. When he gets near her she hisses and does the meow growl thing. She has also taken off after him. Fur flew and she did draw blood one time. I am at a loss. Just last night they were cuddled up in bed with mommy. And this morning it was complete chaos. There was a couple times where they did touch noses, sniff butts, and even sat together in the window. But within minutes she turned back into this demon cat. I'm very upset over this. They are my babies. And she is my girl. So confused. I don't know if she's used the bathroom. But, she has ate and drank. As far as play neither has today as he's on edge because of herm please, I need some advice.

Usually, when aggression comes on suddenly, as if from out of the blue, the first suggestion...is always to visit the vet, to rule out anything medical, which may be affecting her behaviour.

She could have a sore tooth, or urinary tract infection, or sore hang-nail.

Also, it may seem strange, but one member, here, had her one cat attack another,...and it turned out that the 'attacked cat' was ill.

So getting them both checked out at the vets would be wise.

If medical clears...then you look for things like: seeing other cats outside the window, which may upset your cat...and cause re-directed aggression.

Or if you can think of anything that happened recently, that would cause your one cat to be upset...any noises, smells, outside sounds, etc.

When you saw it happen, this morning...can you remember anything that would have triggered the girl cat to get aggressive?