Let's think about that one for a second, shall we?
#1) There ain't any evidence of a Sparda Family Tree past Sparda himself, Eva, Dante and Vergil.
#2) Nero can't be related to Sparda, simply cause the storyline wouldn't work.
#3) I've said this before, but there are several things that point toward Vergil's Devil Form takin' possession of Nero's arm.
-: The markin's on the Devil Bringer are the same as Vergil has on his Devil Form.
-: The Devil Bringer glows blue, as Vergil's Devil Form does.
-: Nero's had a voice sayin' "power, give me more power" ever since he's gotten the Devil Arm.
-: Nero's Devil Trigger has an underlyin' voice/growl, which sounds like Vergil's Devil Form.
-: The Yamato Blade reforged itself in Nero's presence after he kicked Agnus' ass.
-: The Yamato Blade also flew towards Nero's Devil Bringer, which is his right arm. Vergil is RIGHT HANDED.
#4) Although Dante has said he needs to keep Yamato "in the family", he also lets Nero keep it cause he thinks Nero's capable of havin' it to himself.
We will see,
We'll fight until eternity.
Come with me,
We'll stand and fight together.
Through out strength,
We'll make a better day.
Tomorrow we shall
Never surrender.......