I know two sets of identical twins, and even though both sets of twins had the exact same upbringings as their brothers, the four of them have massively different personalities from each other. If you account for identical DNA (which isn't always exactly identical due to changes over time, genes being switched on and off and such - epigenetics) and for similar upbringing, then different brain development, biochemistry and personal experiences is all that is left. Even twins have different spontaneous patterns in how their blood vessels grow and how their neurons link up. So there's always going to be a difference in personality between any twins, even if there are similarities in general.
If you have a kid that believes he was abandoned vs a kid that believes his mother sacrificed herself for him, that would also probably have a massive effect on decisions and viewpoints later in life. My viewpoints on life are different from my three siblings, because the others were actually planned kids whereas I wasn't, and the way I went through childhood as the first born vs the way they went through were pretty dissimilar. My siblings have a very rosy view of life, where I saw a different side to all of that. The knock-on effects from even one small incident can compound in complexity and lead to totally different life paths and attitudes.
Hypothetical Vergil / Dante reversed would mean they turned out differently due the differences in personality already forming - It would be like an alternate universe version of them because there's no chance they would assume each other's lives spontaneously when they were already divergent.
