Okay, so I know I gave Blaine an older sister, and I enjoy the Claire in my head, but I WANT BOMER COOPER. Like, a lot. (*POINTS*)
So I'm going to tweak Blaine's family situation, but I think I can downplay the impact a little. Any character who currently knows about/has met Claire (Kurt, Genny, Santana, Quinn? etc.), has carte blanche to know that Blaine has an older brother who's roughly 10 years older. Blaine still doesn't talk about him much, so even Kurt doesn't know exactly who Coop is yet, but that will all come out next week, mwahahaha. Cooper is replacing Claire outright, because he serves very much the same purpose in my head that Claire was supposed to.
Also, and I've been weighing this for a while now -- I'm going to go ahead, while I'm at it, and retcon Blaine down to a junior. It still doesn't make sense based on S2 Blaine, and I'm not deaging him -- he'll still be 18, just repeated a year when he transferred to Dalton, and probably taking a lot of senior level classes at McKinley -- but it's getting hard to remember that I'm playing him as a senior as we get closer to Kurt graduating, and if I'm reworking his family to include Cooper, I may as well adjust that while I'm at it, instead of waiting until the summer like I'd originally planned. It will NOT affect Blaine's personality at all, it will only add in that level of tension with the knowledge that Kurt is leaving him behind next year.