Did Dante know Nero was Vergil's son?
If I'm not mistaken (been a while since I played DMC4) but I recall Dante saying something along the line of how the Yamato has to "stay within the family" shortly before he tells Nero he can just keep it. Also there is the other things that are right in front of Dante's eyes such as Nero having white hair, being a descendant of Sparda, and his DT bearing a resemblance to Vergil's, so yea I'm 100% sure he knows.