I'm almost positive that the person on the train is Envy and not another Sin. Envy knows what Hughes looks like because he killed Hughes, and it makes sense that Lust would send Envy to track down the brothers.
In order for Hughes to be a Sin, someone would have had to try to bring him back from the dead. The only person in a position to try and bring Hughes back would have been Roy Mustang, and I don't think Roy could have done it because he and Hughes had a fight about that however many years ago.
There's also nearly no doubt in my mind that the child found on the island is a Sin. And if he is, that would make the new kid "Pride". According to the manga, the sensei did try to bring her dead child back to life ... that cost her an internal organ (it's never stated as to which one). That would make the Seven Sins complete:
Wrath (i.e., the Fuhrer)
Sloth (i.e., the Fuhrer's Secretary and the Elric brothers' mother)
Greed (have we met Greed yet?)
Pride (i.e., the Sensei's child)
Either way, though, Lust's statement about Sloth being the youngest Sin is either wrong or misleading. Sloth and Pride had to have been "born" around the same time, or Pride was "born" later and the Sins just don't know about Pride yet.
Makes you wonder, doesn't it?