What surprises me the most is the fact that that Meliodas, who seemed to feel nothing, assured Zeldris they're brothers.Interesting comment from coffin-of-eternal-darkness
That being said, Zeldris must've been quite young to blindly believe what Meliodas said, so Mel can't have said that just before his betrayal. It seems like Mel wasn't all that emotionless at the time.
They might be in the case position as Luffy, Ace, and Sabo, but the genes and the marks...

guess I should've checked the date