I don't know why you guys don't get it, Nero is Virgil's son. He was found in a black blanket with the name nero on it so he was taken in by kryie and her brother. Then when he meets Dante, Dante says " It's got to stay in the family" implying that he is related to dante and virgil. Then there's the fact nero's arm glows blue and virgil has the blue colour. The real question is whether Virgil is guiding and helping nero through his arm or whether he is using nero to regain power somehow and come back from the demonworld.
Obnly according to Morihashi Bingo's AU novel that was written after he left the employ of capcom were any of those true regarding Nero's past. Also, his hand actually glows PURPLE from the top and blue from the palm, and tends to flare red when he's angry. It was originally yellow, but it was changed because the glow effects were not showing up very well. If you watch closely in the epilogue of Nero and Kyrie, it shifts to glow yellow when she takes his hand and doesn't change back. Also keep in mind that eth skin coloring of the scale plate sthat encase his demon arm are red and blue, because his character is intended to blend Dante and Vergil.
Also, I can point out how compendiums and Artbooks are starting out outright contradict the games they are suppose dto expand on. The Hyrule Historia is notorious for this in the "official Zelda Timeline" as much of the descriptions of events fly in total contradiction to the same events actually shown and/or explained in the games.
So just because there are notes in the new art book from early planning or Capcom just decided to be lazy and slapp in "Nero = Vergil baby because we're so creatively bankrupt" does not mean I will ever just go "Okay, so Vergil had a kid at ten." Because Nero being a quarter breed cambion and Vergil mating with a human creates so many issues in on their own. Nero being a 3/4s hybrid, I could better believe, but again, Capcom won't explain it. that meas they never really actually planned it out and the "Nero is a child to Vergil", which also implies MORE kids somewhere", is a quick copout.
And no, retconning the timeline AGAIN will still not change my view. Dante was 29 in DMC4, as it took place a year after dmc1 where he was 28, that is word of the directors and WRITERS at capcom