My brother and I both have 2 kids, both the elder and the younger are different within 2 weeks, the funny part is we didn't plan this. It just happens.
Both big kids are different within 2 weeks, the younger kids original is different by 2 months, but my brother one pop up earlier, 2 months earlier, so become 2 weeks different.
The younger kids are still young so can't tell for now, but both the big kids are quite close, but you can tell that the 2 of them are so different, it might be related to the growing up background, how we teach them individually and the environment.
One of the interesting issues happen last night during my bid kid birthday party, so the cousin has personally chosen a present for my kid, a transformer toy robot that he likes. During the whole party, he keeps wanting my kid to open up the present so that he can play. After my kids open up the presents, the cousin starts to play the toy that he chooses to gift my kid. It's a bit funny for the whole situation yet I can't understand what the kids are thinking?
But lucky that my kid is quite ok with it, he did mumbles like isn't that a gift for him, but still ok to let the cousin play with it most of the time. The cousin might feel like wanna give his favorite type of toy to my kid, yet he likes the toy very much which explain why he can't stop playing with it, even though is a gift for my kid.
Anyway I think is pretty awesome that grew up together with someone close, It should be a pretty good memory for them.