Ok, let's see. As far as I'm concerned Nero only has Virgil's blood in
the devil bringer, because the devil bringer is Virgils right arm.
Notice how when they got 'Virgil's' blood they got it from the devil
bringer?
And Dante and Virgil aren't in there 20's. There exact age is unknown as far as I know, but they have lived for quite along time, that much I know. They only look young because the aging process is different for them being half demons.
And Sparda definatly had only 2 sons as far as we're concerned, cuz if Nero was another son of Sparda he'd know because he would be half demon and hence age differntly. But sinbce he'd probably only be young (say 19) then that would mean that he'd have to be conceived 19 years ago, and Sparda hasn't been seen or heard of for much much longer than that.

Blackness covers all I see and all I touch
No feeling of pain just sorrow
My hand loses grip of the sword I held
And I turn to become a true shadow.
(\-M-/) Muah ha ha...Fear the Bunny King!!!
(='.'=) And his Holy Hand Grenades!!!!!
(")_(")±±±±± => Holy Hand Grenades (sort of)
Breaking News: It would appear that the Bunny King failed maths in Primary School, the test in question involving counting to 3!!! XD