NerosDevil, you are basing your thoughts way too much on facts. And facts seldom work in videogames, especially in games such as Devil May Cry.
And inheriting something doesn't mean you have to be born with it. Nero can inherit Sparda's powers some point in his life, and so he did when the Devil Bringer absorbed Yamato's powers.
I don't remember Sanctus ever saying anything about Nero being a descendant, only that he inherited Sparda's powers. Could you please send me a cutscene where Sanctus says Nero is a descendant of Sparda?
But even that wouldn't prove anything. Let me give you an example. I don't know if you have played any Castlevania games, but in Aria of Sorrow this guy named Graham inherits Dracula's powers. Yes, he was born as a completely normal human, but he INHERITS Dracula's powers. And guess what? He even gets Dracula's bloodline in the process. So I guess that's pretty much what happened with Nero. :D