A descendant of Sparda's blood does not specifically mean Nero is Vergil's son. Perhaps Nero does have Sparda's blood running through him for other reasons than him being an actual descendant of Sparda himself. Maybe Sanctus was just trying to confuse Nero even more than he already seemed to be about his background/past. So taking that into consideration, we're going on the word of a deluded/deranged old man who sacrificed himself in some demonic ritual to be born again as a demon. Can we trust Sanctus as far as we can throw his old demonic ass? Perhaps not.
But, let's go on the assumption that he was telling the truth about Nero. Where does that lead us? Well, as said before, Nero can't be Vergil's son because it doesn't work out on the time-line. It also means he can't be Dante's son either. BUT, it does not mean that Nero can't be an actual son of Sparda himself. A younger brother to Dante and Vergil, if you will. And furthermore, it does not mean he can't have a blood relation to any other beings that had Sparda's blood running within them for whatever other reasons. Remember, there's a lot of 'this happened and that happened' when Sparda was around, but his life seems to be Legend/Myth. I believe he kept to himself, and very few people really knew what was going on with him. He was probably very secretive, I'm sure he had to be for protective reasons. With that said, there's no telling who also holds Sparda's blood in them. Whether they are part demon, half demon or whatever.