Someone once said to me, "Siblings are your proof," meaning that your brothers and sisters are witnesses to what happened in your childhood. You know, the ones who will back up the story as the kids saw it, not the parents' version. The ones who know what really happened on that vacation, or that family party.
I agreed, but that's because I've always gotten along with my sisters. I imagine that there must be many families where siblings aren't proof, but rather those liars who don't really remember what happened, or who are just trying to make me look bad, or are sucking up to mom and dad. No matter which it is, though--love, hate, resentment, jealousy--you're tired to your siblings like you are to no one else, even your parents.
Alexander Waugh's "The House of Wittgenstein: A Family at War" tells the story of the Wittgenstein family (hence the title), meaning the philosopher Ludwig and his three sisters and four brothers. One brother definitely committed suicide,while two others died in murky ways that suggest suicide; in fact, there were an inordinate number of suicides in the family. If you ever had any doubt that that turn of mind is hereditary, this book might sway you to the yes side.