You never know the possibilities, so speculating is just going to cause arguments. Age wise, Nero could not have been Dante OR Vergil's son... Furthermore, how come we never hear in the scenes throughout the entire dmc series that Sparda had 3 sons? It tells us he had two, but it never states that there was a third. And due to Sparda's love for Eva... Nero would defo be Eva's child if he were to be a son of Sparda.
Lastly...
He could indeed be a son of Sparda, but I'd like more logic behind him than that... That seems kind of "Oh look, that's an easy way to explain Nero... Let's go with that".... It doesn't feel like a strong enough explanation as to his power... And even so, how does that make him have some weird demonic right arm? Vergil and Dante don't have one, so why would Nero be different if he were a brother?