Sorry to hear you are having a hard time!
FWIW my DD's (3 and 4) are best of friends nearly all the time. What's worked for us is to try to teach them empathy and to explain behaviour/consequences/feelings very clearly to them from the age of your DS's.
This means that when one is being nice to the other you don't praise as in saying "good boy", but in pointing out "look how happy you made your brother, he is smiling"; so the "reward" isn't selfcentered but learning that your actions have consequences and that it feels nice to make other people happy.
Likewise, when they fight etc. the "punishment" isn't selfcentered in form of naughty step, being told off etc, but a clear NO and then pointing out that the behaviour is hurting the other person, making them cry etc, possibly even explaining that they would be upset themselves if the brother was behaving like that towards them.
If things go pearshaped, the "worst punishment" in our house is that DD's are told to go in separate rooms and that they aren't allowed to play together until they can play nicely, thus the "reward" is to be allowed to play together.
Hope this makes sense and hope things get easier for you soon!