I have a buddy who never participates in a threeway unless the other two guys are brothers.  Here's what he tells me about the varieties of brother sex he's enjoyed over the years:

Half-brothers: My buddy swears that he can tell by the taste of cum if two half-brothers share the same father.  He says that if the brothers came from the same sperm donor, their own cum tastes similar, but if the brothers have different fathers then their cum tastes different.

Step-brothers: My buddy gets a perverse pleasure out of step-brothers, because even if they are technically singletons and have no biological siblings, they can still participate in incest courtesy of their parents' marriage.  The same goes for adopted brothers: even if they don't have any blood relations, they can experience incest with their new brethren.  He says that the "taboo" nature of brother sex gets even step-brothers and adopted brothers extra excited in the bedroom.

Brothers-in-law: My buddy also loves threeways with a man and his wife's brother.  In his experience, a lot of men love their wives but also secretly desire to fuck a masculine version of her.  In the best cases, my buddy says, the husband fucks his wife's fraternal twin.  He says that such encounters are wildly passionate beyond belief.

Twins: You'd think that identical twins would be the ultimate in brother sex, but my buddy actually doesn't seek them out.  In his experience, identical twins in the bedroom are either so into each other that they totally ignore the third guy, or they get competitive with each other and the sex becomes overly aggressive.

Is it hard for my buddy to find pairs of gay brothers?  Not at all, he says, since it's common for more than one brother in a family to be gay.  Also, he says that it's common for one brother to be gay and the other simply game.