There's no denying, though, that Shakespeare prefers elder to younger brothers. Look at his list of younger-brother nasties: Edmund, Claudius, Richard III, Antonio (as in The Tempest) - actual or would-be usurpers all. Add Prince Henry's brother John, who's loyal but in such as way as to congeal the blood, and it's a pretty unappealing bunch. The only bad elder brother I can think of is Oliver in As You Like It, and even he's counterbalanced by Duke Frederick - yet another usurping younger brother, come to think of it.
For the sake of completeness, I should mention loyal Marcus Andronicus - but I've never quite been sure whether he's older or younger than Titus. I'm sure there are others I've forgotten.
So, does this mean that Shakespeare (an eldest brother himself) had some kind of party loyalty to his confreres? Perhaps, but it strikes me as interesting that his sympathies are so often with the primogenetically-advantaged, where folk tale is so often aligned the other way. Admittedly he bought his plots wholesale at St Paul's churchyard, but there were other plots he could have got, were there not - had he so chosen?