I'm late but I'll jump in anyway...
My brother and I were always really close; he was born when I was 2 years old, and I thought he was my baby. I sang him to sleep every night and begged to be allowed to change his diaper.
As we got older, I would do other things to take care of him, like cleaning his room while he watched cartoons as a surprise to make him happy. We argued, but I don't think either of us thought of it as arguing; I remember being frustrated when my parents would get angry about what I saw as simply animated discussions or debates.
Something that may have affected our ability to get along, however, is I suspect that he's also an Aspie; he was almost diagnosed as such, but because it was during a time when the school had essentially said he couldn't go back to class until a professional declared him not a danger to himself or others, they didn't worry too much about identifying what *was* wrong and instead cared about saying that whatever it was wasn't a problem. He's now about to graduate from high school and any social struggles have pretty much disappeared (so maybe I'm wrong), but he definitely took years to listen to my mother's instructions to look at people when they talked to him, to say hello when greeted, to ask others how they are; he was obsessive about dinosaurs, then about guns (which played a part in the school requiring he be checked for dangerous tendencies), and now about wilderness... I could continue, but the point is that I think he's an Aspie (I definitely see a lot of Aspie traits on my dad's side of the family, so it wouldn't be surprising).
Maybe because of those shared traits, I always spoke for him; I somehow knew exactly what he was trying to say when no one else could follow his train of thought, and in some ways I know my parents believe I added to his problems because, with me there to translate, he didn't have to learn to speak for himself. We always did everything together if possible, even if we had classmates around us with whom we could choose to play instead. I feel so lucky to have been so close to him. Of course as he got older, we grew apart, but even when I was a senior in highschool he loved to come talk to me at lunch, and he still brags to my past teachers that I'm his sister. I'm not sure we would have had such ease, though, if I'd been higher on the spectrum or if he'd lacked Aspie traits. As is, it was like we had our own way of communicating and I was the one who could translate for the rest of the world, but if we hadn't had that connection or if neither of us had had the skill to translate I suspect there would have been more frustration, resulting in tensions and arguments.