I have 2 male Siamese cats, they are brothers, one of them has become very aggressive toward the other, ie: hissing, attacking for no reason, starting fights. I'm worried that something is going to happen to the other one but don't want to give up the aggressive one because he's my daughters first pet. What might be causing this? They are not fixed but have appointments next week to be done.

Wait until after they are neutered. That is probably your problem.