The series doesn't give us a lot of information about many aspects of daily life, including civil law. If we assume a property system something like Elizabethan England, though--a system dominated by elder sons, with an obsessive concern about "legitimacy"--it could explain a lot.
Blake's easy sense of command, for instance, could stem from being raised as a privileged older son (we know that he had a brother and sister--or at least thought he did; the whole thing could be a fabrication either by the Federation or the Freedom Party--he could certainly be the eldest and designated heir).
If Dev Tarrant is the oldest brother, naturally his attachment to the status quo would be much greater than his younger (half?) brothers Deeta and Del, who would not inherit and would therefore have to seek their own fortunes, possibly outside the system.
As already mentioned, Avon's presentation as a rather secretive outsider sounds a lot like Shakespearean figures like Edmund, Don John (and Richard III, whose claim to the throne requires getting several of his relatives out of the way). Instead of "hey, I'm depraved on account-a I'm deprived!" (which could be the motto on Vila's coat of arms) one might suggest that Avon IS a bastard and therefore a bastard. He certainly isn't overly impressed by the sacredness of marriage vows.
-(Y)
PS--readers of my fics will know that the last suggestion is not made seriously--Avon's parents are not only too happily married but too respectable for this to be the case.