Could Nero be a reincarnation of Vergil?
He seems to be young, and DMC4 takes place some time after DMC1, where Vergil died. Notice his name is similar to Nelo Angelo, Vergil's name as Mundus' servant, his devil trigger appears to be a ghost version of Vergil, and also, he wields Yamato, Vergil's sword. Seeing as his origin...or anything about him really, is left untold, this could be the explanation for his similarities to the sons of Sparda. Either that, or he was posessed by Vergil's spirit at a young age. I know, i haven't played the game yet, so maybe something is said in it which already discredits the theory, but seeing as Vergil is a fan favorite, as well as an important part of the story, yet is deader than dead, this is Capcom's way of kinda bringing him back.
Either that, or Sparda had a few more adventures besides Eve. :P