To be short about it, Ciaran and his younger brother were born to an Irish woman named Saoirse (said 'seer-sha'), and a Pakistani man they've never met. Odd a combination, yes, but welcome to Rukongai. Lol So father fucked off somewhere and mother eventually died; technically they may not entirely be orphans, but Ciaran just ran on the assumption they are. (Side note, if anyone wants to play dad, go for it. That'd be a trip.)
Brother was a bit too young to manage much of anything, so Ciaran kind of started filling the role of dad, mom, and older brother, all at once. Things were rough, but they had each other, and that was the important part. Most likely, brother followed Ciaran to the Seireitei when he left Rukongai, so he's most likely going to be a shinigami.
Now, there's not much set in stone here. The only thing is, younger brother probably has the same Ciaran-like inability to lie and also probably has his hot temper, too. Beyond that, s'all up to you. We can banter and brainstorm, or just post together and see what happens. Ciaran is, and has been, dating Kurogane Iname, one of the big-name noble guys in the Seireitei, for quite some time now, they ended up attending the Academy at the same time and got to know each other that way, so that's a thing to know.