I would imagine the Centauri are a sort of constitutional monarchy; hereditary emperor if there is a clear line of succession, elected (or selected emperor if there isn't a logical successor (or a regent if the appointed emperor is underage or unable to serve at that moment). Sort of like England post the Stuarts. Centaurum members might be elected, or they might inherit their seats. Vir is a Senator when Londo is Emperor; sounds more like elections. I never got the impression Vir's family was that well-situated.

You know, I really love Vir as a character, but excusing his silence in the face of mass murder due to personal loyalty is a little tough for me. Silence because of feeling helpless and overwhelmed and uncertain seems more Vir. And who knows if at this point he really thinks of the Narn as fully human? Most Centauri don't seem to. Refa tries to make the point that this outpost is a military one, but I don't think that's proven by any means.
Vir comes to understand that we all have choices, even when we don't think we do, and he works his way back from his personal abyss. None of these characters is pure and innocent, and that is why I love B5.

My apologies to those who worship Valen! (But Sinclair is still nuts. And a good thing too. Sometimes you have to be crazy in order to get done what needs done.)