As the oldest sister of three girls in my family, being the big sister has meant a lot of different things growing up. It has meant that I have parented (way too enthusiastically, according to my mother and probably my sisters), I have protected, I have led, I have supported, and I have learned the exact ways to infuriate my sisters. In recent years it means I have become good at smoothing things out, helping iron out family worries and calming angry siblings down (until it comes to me, and then sometimes it falls apart).
It seems like a good preparation to be a parent; I definitely have that same pride when either of my sisters succeed and the same worry and sadness when they have to deal with hardship and heartbreak. It's better than being a parent though because I know I can count on them for the same things they get from me; someone to commiserate on the jackassery of my husband or boss, torture about every embarrassing thing I ever did or wore growing up, and unquestionable love even when I am the worst version of me.
They will never unfriend me, lose my number, or experience a major life event without me being the first to know. I have only one friend for whom this can be said, but I have two sisters for whom this will never change, no matter what I do.