Here's one of the many things I love about Beholder -- it makes me believe all sorts of pairings that initially seem highly unlikely. I'd never have thought of Aberforth/Sirius, but as you write them, they make total sense. Both are outsiders despite their "pureblood," inner-circle status; both are men who have been broken but who are yet strong enough to put themselves back together -- a little maimed and misshapen, perhaps, but still together. Both are like wizarding versions of Holden Caulfield, raging against the phonies. Both are sadder, hopefully wiser men, yet still adolescent enough to enjoy snickering about big knobs. In short, I really your crusty, plain-spoken Aberforth and your subdued-but-still-Sirius Sirius; IC, the both of them.
Lines I liked: It wasn't until Sirius would order something ridiculous like a martini, or a gin sling, that Aberforth would speak. And that was only to kick him out. Haha!
After Voldemort vanished, Aberforth had a hard time believing Sirius was responsible for those murders. But then he remembered that anger. Anger could drive you to complete madness. Ouch.
"I can hardly remember why now. Probably for a laugh. Everything was a laugh then. Everything isn't so funny now."
"Things weren't funny then, you were too young to realize it." Perfect.