A couple of months ago, I heard a radio program on sibling relationships, with a psychoanalyst (they still get on the air in the US?) and a clinical psychologist (who supposed that self-reported sibling relationships were data). Both of them came from the position that sibling-relationships were understudied and poorly understood.
To which I say: hurry up. My older sister (I was 3 of 3, she 2 of 3) was for a very long time the absolutely most important person in my life, able to elevate or devastate me with a single word. To this day, I worry more about what she might say to me more than I do about my parents' opinion.