Cats are fighting :(

I have 2 cats - they are a bonded pair (brother and sister), spayed / neutered.

One is a boy one is a girl. The boy likes to go outside and the girl doesn’t. Today, the boy spent awhile outside, and when he came back, the girl is really upset at him. She is growling and hissing at him, and she keeps trying to fight him!!! They fought a little bit before, but never for this long or this much.

They generally get along well … I don’t know what happened (is there a weird smell he brought back with him?) or how to stop it!!!

Any advice or insight is welcome!