Are they ever actually called the Moya Brothers? The ending credits lists them as the Moya-nin (モヤ忍, Haze shinobi).
If they aren't called brothers, it is doubtful they are actual brothers. Although the leader is called big brother (アニキ, aniki) (which is not a name, by the way), the other two are simply credited as Moya-nin underlings (モヤ忍 手下, Moya-nin teshita). Aniki is often used by gang members to call their leader.
Which brings me to my next point, I haven't found any evidence of the two underlings' names actually being Mo and Ya.
The two underlings are voiced by Hideo Watanabe.
The mark on their forehead protectors isn't a question mark, it is a stylised haze cloud.
They might have been pathetic shinobi, but from what I've seen (which isn't a lot, i just skimmed through the episode), they aren't completely without skill. Those poisons they used made quite well.