Tigermoth, my brother & I were so close as children that we would choose to spend time together instead of with friends. Ours was a very insular relationship. He left home (aged 18) the weekend I got married because he couldn't bear to be there without me. (Think it was that I shielded him from mum & dad, rather than missing me )
He spent several years in squats. He would do nothing for his family, but his friends got everything he could give. We had him live with us for 2 months,nearly wrecked my marriage & he threw it back in our faces. I've tried to heal the breach & we've visited him in every "new" flat he's had (a huge number), but we've been living in this house for 7.5 years & he has never been here.
We asked him & his wife to come away on holiday with us- (for free)- they turned up at the caravan park, he made some snotty comments about the decor of the caravans- they stayed ONE night then went home. He is a complete snob & won't put himself out for any member of his family, especially if there is a friend to see instead. he was supposed to take my mum to see his DD on her birthday but decided to go on to friends after & made her drive herself...
He was there when my father died & helped me with mum; but he alienated our grandparents years ago & was the missing link when we needed him for them. We take my mum on holiday with us- he says he's "not that stupid". I'm dreading the time she becomes old & frail because the burden will fall on me. I just hope my own kids will be there to pick up the slack.
He can't stand my kids & won't spend more than a few minutes in their company. When they were little his own DD (same age) would roll around him & climb on him & he'd be all lovey-dovey, but mine would try it & he'd immediately be "get off". He speaks ABOUT them with contempt & doesn't speak TO them at all.
DHs brothers are fairly human but he's seen them once in the last 7 years, despite the fact that one lives 10 minutes up the road & the other about 25 miles!
I don't think the non-conditionality of parental love necessarily applies to adult siblings.