Heh, I'm also an only child, and likewise often think of the worse-case scenarios in terms of what it might be like to have a sibling (a la "Jacob, Have I Loved," etc. Even with surrogates, however, I don't presume to say I know exactly what it's like to have a sibling *cough* HP! *cough*.

That is an interesting thought re: adults, I have often felt more comfortable making conversation and such with them than people my own age when I was younger, maybe you are right as to why...