I get along with my parents. I've got a younger-by-a-few-years brother, and I'm the sort-of-black sheep who bends all the rules so by the time he can do something, it's okay.
I've always been a good kid, more or less, just because I am. Sometimes my parents want me to not play video games so much, or sleep earlier, or whatever, and we have a polite disagreement over it, which is fine by me, because I Love Arguing and usually win. But we don't do that often, and there's never been any serious fights.
With that said, I get the feeling that I'm not as close with them as other children are with their parents - I've never felt I can rely on them for emotional support, because they have a tendency to mock things and to try and interfere in stuff that isn't there business (and do it badly), so I'm never really open with them, they're never really open with me - they hid their separation for two years after it was official, but I'd seen it coming for 5... So I get along with them, but it's not close. That said, I'm incredibly fond of my rather large and close extended family, so when I think of family, it's the whole collective, not
As to the OT, I think having a sibling gives you more options when dealing with your parents. Because it's important early on to be fair to both siblings, the idea of negotiating fairness eventually worms its way into the parent-child relationship, and puts them on a more equal footing. With the different dynamic, you can shift blame around, look elsewhere for support, have someone to have tensions with OTHER than your parents. Like you said, the two children means that each one can spend some time being unscrutinized, I imagine that it relieves tensions. A lot of the only children I know got away from home as fast as they could.