Mmmmmmmm, I have a different point of view when it comes to that. Vergil and Dante are twins born on the same day. As much as Vergil may want to believe, there is no older brother, there is no younger brother. How much wisdom and knowlegde are you gonna gather in...what? Five minutes? Three minutes?
Soppose their mother never told them who was born first, they would never know. From what I see in twins, (In real life too.) They tend to switch roles,like whenever the other may need it. For example, I think it was Dante who was playing the big brother role around the end of the game. Not because he whipped Vergil's a$$, but because he was the only one that seemed to have common sense and was stable at the time.
Dispite the fact that it didn't go all that great in the end, Vergil needed Dante playing the big brother role then and there. Just like Dante needed Vergil playing big brother for his awakening.
...Ahem...Please excuse my lecture...Hmmmmmmm, I should make a topic on that.