I have a sister who is 5 years younger than me and a half-brother who is 12 years younger than me.
My sister always has been and always will be my sister. I have never lived with my brother, and hardly ever see him. The last time I saw him was over two years ago. This is because of my dad, not my brother. My brother can be a bit of a brat sometimes but he has been brought up by my father so that's not really his fault. My dad's partner also has 4 children from a previous marriage. I have met them once or twice. They live closer, so I think my brother sees them more than he sees me and my sister.
I know 12 years isn't as big a gap as the one you're talking about, but I do find it very hard to think of my half-brother as part of my immediate family. He feels more like a cousin that you don't see very often, and I feel awful for feeling like this. But I don't know him. I don't know what he likes or doesn't like, I don't know any of his hobbies. I've tried to write to him but all I get in response is a short email from my dad's account.
Sorry, this has turned into a bit of a rambling rant. I think my point is that I have a hard time thinking of him as part of my family. I think this is down to my feelings towards my dad rather than the age gap. I imagine I would feel differently if my mum had another child. That sounds horrible but it's how I feel and I can't change that
