2. How does everybody know that Envy isn't pulling the wool over their eyes? He's a shapeshifter and often takes on different personas, so obviously he's good at acting and lying. He has no reason to even tell Ed if it were true. And, as Ed promptly froze long enough for Envy to stab and kill him after Envy told him, it is very likely that it was a ploy for Envy to get an opening to kill Ed. And I'm sure it would be right down Envy's alley to mess with Ed mentally like that.
3. We've all seen it: the bad guy confesses some sort of close familial connection with the protagonist, then maims/kills said protagonist. For once, can't they just be lying? I mean, if I were a bad guy I would totally make up a lie like that just so I can off my arch-nemesis. I also wouldn't actually tell them unless I utterly had to if it were true. Seriously, do you honestly think it would be beyond your average bad guy to say that just to mess with the protagonist?
4. Of course, how do we know that the face Envy wears normally is even his real face? A comment by Lust early on indicates that it isn't. He could be anyone for all we know. Heck, he could be related to Ed anyway, but as Trisha's great great grandfather or something like that. He could be related to anyone. But since his reply to Lust's comment was that he liked looking young and pretty, as a human he was either old and hideous when he died or pretty when he was young.
5. Of course, how do we even know he's a guy to begin with? With his voice he could go either way, plus he can take both male and female forms, demonstrated when he took the form of Ed's older brother to kill Ed and of Gracia to kill Hughes.
Of course, this argument is strictly anime. For some reason, I find it more believable that his true form is a billion tons and sinks into the ground like a glacier and leaves a dent then he is Ed's older half brother. His true form is incredibly sad as well-I believe Hiromu Arakawa has some explaining to do there.