Quote 1
"My brother Albus wanted a lot of things," said Aberforth, "and people had a habit of getting hurt while he was carrying out his grand plans. You get away from this school, Potter, and out of the country if you can. Forget my brother and his clever schemes. He's gone where none of this can hurt him, and you don't owe him anything." (28.43)
Aberforth bitterly notes the tendency for people to get hurt by Albus's schemes, even if they're for the greater good, and doesn't want Harry to willingly give himself up just to finish the path that good ol' Albus started him on.
Quote 2
"Funny thing, how many of the people my brother cared about very much ended up in a worse state than if he'd left 'em well alone." (28.55)
Aberforth observes a curious phenomenon about his brother's love – it often ended up hurting people, the very same ones he professed to love. Well, love does hurt, after all.
Quote 3
"[Albus] told me I had to finish my education and he'd take over from my mother. Bit of a comedown for Mr. Brilliant, there's no prizes for looking after your half-mad sister, stopping her blowing up the house every other day. But he was all right for a few weeks… till [Grindelwald] came." (28.69)
We finally learn what happened to the Dumbledore family – it seems that young Albus was ready to sacrifice his early career to take care of Ariana. But his was not entirely a willing sacrifice, as demonstrated by his easy distraction by Grindelwald's charms.