My two cats are jealous of each other!?

I have two cats. They are brothers. I raised them both from little kittens. They're about 5 years old now. And they hate each other! They fight all the time. And now the only time I can get them in the same room is when wet food is involved. Also, it seems like when I'm holding/petting one, the other one is always around and glares at me. When they were kittens they loved each other. Cuddled together at night. Shared me without a second thought. The older they get the more they seem to become jealous of each other. And yet as much as they hate each other, we've been unable to separate them. I gave one to my sister and the whole time, he was crying out looking around for his brother. Likewise, the one I kept did the same thing. My sister couldn't bear it and gave him back to me. Now they're back to fighting again. Is there any way to fix this? They only like each other when the other isn't around.