DMC4 spoilerrrr
That was never actually confirmed by Capcom proper, just hinted at in the novel prequel of DMC4, and "confirmed" by a member of Capcom USA who claimed to have some vague connection with the game's localization. Sounds rather shaky if you ask me, when you consider that Capcom's official stance on all the Devil May Cry novels so far has been that they are licensed AU-fics at best, and absolutely detestable non-canon entries at worst.
Even so, only Sanctus even believes it in the novel. Nero thinks the story is a load of crap, and Dante considers the prospect of Vergil sexing ANYONE to be an outright impossibility. And so should you, given what we've seen of the man.
So yeah.
The prevailing theory I've seen on the net seems to believe that Nero is a more distant relation to Sparda, like a descendant of another son of Sparda born during the Dark Knight's indeterminate amount of time spent on Earth, with Nero's blood being "less perfect" in Sanctus' eyes because more generations had passed for Nero's family and so he was biologically more Human than Dante and Vergil, who are direct offspring of Sparda, rather than a distant descendant.
Granted, there's not even a semi-canon AU novel to draw on here, but it's not logically impossible either. And it makes more sense than Nero being the son of a man who only gets a boner for power rather than weakling human women.
Just saying.
And that's my two cents.
Have at you.