He was shallow! He was a shallow shallow man! This is what I get for thinking about things. Details happen.
Delicious details. Wonderful details. Wynston has become more and more interesting as you've written him and I think that's great.
Eee, history lesson!
Though I'm of Irish extraction I know very, very little of my own family's history.
Oh wow! We do get around, don't we? There's way more people of Irish extraction living around the world than back here in Ireland. Waaaay more. As in, multiplied several times.
I will say that Wynston's parents and at least some of his other siblings maintained closer ties to the old community than he did. I don't know how realistic it is that they had the option to stay and chose to go anyway. Wynston personally was (economic reasons/more economic opportunity driven by parental choices) + (screw you all I'm leaving and assimilating as hard as I can because I hate the old country and, as a running theme in my life, I am prone to the sudden and complete cutting of ties.) He's definitely glossing over the fact that he had a community of social equals.
The above account is all the thought I've ever put into his family; I've never really thought about his siblings and how they would have adapted. If I were really awful I would dub my Chiss BH his big sister...think, think think *taps head, Winnie the Pooh style*...
Maybe I'm picking up on what you wrote wrongly, but I got the impression that he hated his family for accepting their place in society and not fighting against it. On reflection, that really fits with his personality overall. He's never been one to roll over and just say "this is how it is, I can't change it." Everything he does post-chapter 3 seems to be part of an overall attempt to make things better, whether on a huge galactic scale or in Ruth's life in this fic.
As for the possibility of making your BH his sister, I wonder why you say "If I were really awful..." Would this be bad for Wynston? Would this cause him discomfort? Would this possibly... somehow... end up giving him (wait for it)... an air of noble tragedy?