Sudden Aggression between Lab Brothers

Question
My brother has had 2 wonderful yellow lab brothers (same age, same litter). He's had them since they were about 12-14 weeks old. They're now 3(?) and all of a sudden they HATE each other. Can't be in the same room. He has to separate them but as soon as they're together again, they go at it again (he's very concerned). They've never been apart, They've slept together in the same cage, they've always had a back yard (good sized) with a dog door to come and go. But seldom EVER leave the yard for a walk, run or visit a dog park. A few months ago my brother moved in with his girlfriend who has a dog and they've gotten along GREAT! Running together in the yard (again very good sized). I don't know if he's changed dog food brand. He doesn't give them people food ever .... these dogs are his 'boys' and he just can't believe this is happening.
Thoughts?
(they're neutered... have been since they were very young)
Much Thanks
Gerry

Answer
I can suggest that the third dog, the girlfriend's newest addition to this "pack", might have precipitated a crisis in "rank" in this pack.  Without observing I can't guaranty this is the cause.  However, I have seen male littermates who developed strong aggression problems toward one another seemingly overnight.  The age of 3 is an adult threshold in the domestic dog and we can often see new behaviors demonstrated at that time, especially rank problems.  This behavior problem needs to be seen by a credentialed behaviorist (and I don't mean a dog trainer with some make believe titles) in order to be properly addressed.  I have successfully rehabilitated adult litter mates' relationships but it takes several visits and must be done carefully.  I suggest you contac a Veterinary college in your area and ask for a referral to a legitimate expert in dog behavior.  In the meantime, your brother should CLOSE THAT DOG DOOR and make ALL dogs EARN going out, coming in, by doing a simple obedience behavior such as "sit".  Feeding should be supervised and the two dogs need to be fed separately until the top ranking of the two is determined by an expert, at which time your brother will feed him first, allow him in and out first, in order to set "rank" in the mind of his litter mate.  It's a tricky problem that needs to be addressed soon or it will soon deterioriate to the point where these dogs cannot live together.