Thing is, we have no concrete proof to say that Nero isn't related to the bloodline, sure, the fact that he is happens to be speculation, Sanctus only made a few fleeting comments in relation to the fact, but then again, Sanctus NEEDED a person who was both a decendant and shared the blood of Sparda.
At such an early stage, I don't think it's fair to rule out any possibility unless it's entirely rediculous, such as "Nero is a time travelling cat/slug hybrid with Sailor Scout powers."
While he may not be a technical "Son", there is every possibility that Nero could be a Step-Son or something like that, I really don't think anyone has a leg to stand on denying such a possibility at such an early stage, unless someone can produce hard, sourced evidence to the contrary, then no one can deny the possibility of Nero's relation to the twins.
Capcom has confirmed that there are only two sons, many times actually, but it still doesn't rule out the fact that Sparda could have fathered a child with someone else, or something along those lines.
There is also an extreme likelyhood that Nero is more deeply connected to Vergil, as Vergil is the only character who had such an intimate bond with Yamato, yet Yamato reacted in a strange fashion with Nero, as if returning to it's owner.
There has also apparently been news of Vergil's appearence in the next Devil May Cry, chances are, we'll learn more then.