I don't get on with DB at all. I think growing up we weren't close as we're 4 years apart. It's just a bit much. We could never bond in high school as he was just finishing when I started.
I'll add that we have totally different interests and personalities and I actually really dislike the stuff he likes. Sport obsessed and awful taste in music. His hobby bores me senseless. Still, if we'd been closer in age maybe we would have influenced each other's taste. A 13 year old boy hardly likes the same music as a 9 year old girl. Or 16 and 12. Same with books, friends or anything really.
I'll also add that our parents were rubbish. DF left when I was 12 and DB 16. DM wasn't up to the task of single parenting, especially with a very angry teenage boy whose dad went from one of his only companions to pretty much NC overnight. Still I could have forgiven him the aggression and making my teenage years awful as I understand how hard it was for him, but there's nothing there to actually build a relationship on now.
If he and DM do get together she still goads him and he still calls her stupid etc so I avoid family get togethers at all costs. The last time I saw him was about 6 years ago when a meet up was sprung on me. Since then he called when DS was born almost 4 years ago, and texted when DDs were born almost a year ago. I don't foresee any contact for years to come.
I kind of dread my DM dying just because I'll have to talk to him to arrange a funeral and all that. I know how awful that sounds but it's true.
I thought long and hard before having another baby because I hate the thought of my kids being like this. I should mention here though that DP and SIL are super close even though they have almost nothing in common and very different personalities. That swayed my decision.
I really do think it's 90% down to good parenting. Had mine been better able to facilitate a good relationship I would have been very willing to make the effort with DB. I think the love is naturally there for siblings and can get chipped away at if the parents don't protect it. I'm interested to see if others think this is the case. I am desperately envious of people who get on with their siblings and would of course like that for mine too. Just how do they do it?