It seems a lot of people answering in the negative are those that have male siblings of their own. I fall into this category and admit my first thought on the subject is that it's not something I'd particularly want to see.
Maybe that's just a knee-jerk reaction from personal associations, though.
I wouldn't mind a scene with two cute guys with some sort of resemblence to eachother having sex and there being some dialogue that suggests that sort of relationship. But, I think the "talent" would be the main factor in me watching, not necessarily the brothers concept.
After all, how many mainstream porn themes are predicated on the "daddy's little girl" scenario (just as, prolly more, of an exploitable relation IRL)? I'll gladly watch the scene if it's a performer I'm into, but it's not a genre I really have a specific fetish for. And admittedly a lot of that stuff is gonzo and the storyline is more laughable than serious.
Even look at Playboy and the like, and how well twin/triplet/etc spreads go over. Obviously a bit different from full-on hardcore porn, but they're naked, its sexually suggestive, and they're related.
Certainly has its market.