In some ways, siblings and especially sisters are more influential in your childhood than your parents… Deborah Tannen, interviewed on NPR, on relationships between sisters

I. My Sister, My Self: The Surprising Ways that Being an Older, Middle, Younger or Twin Shaped Your Life (2006, then available in paperback 2012) by Vikki Stark, MSW 

Family therapist Stark’s research involved interviews with 400 females ranging in age from four to 95.

Selected quotes from her interview at Commitment Now. (Note: The url seems currently inactive.)

…(W)e assume that a sister is just one-step removed from your ‘self’.  It goes without saying that a woman’s sister should be the person to whom she can turn at any time of the day or night. She knows you better than anyone else and would make any sacrifice on your behalf.

In a nutshell, women who are older sisters tend to suffer from the need to make everything right for everyone and have trouble letting others take care of them.

Middle sisters may seek their identity through their social life outside the family as kids, but as adults, they often become the backbone of the family. Among their friends, they are good mediators because being both older and younger sisters; they can easily see two sides of an argument. They’re often more black-and-white, practical and solution oriented than their sisters.

Whether the youngest one is the adored, spoiled family pet or the overlooked afterthought, the hallmark of the youngest sister is powerlessness. As a result, as an adult she often either becomes fiercely independent (to prove that she’s someone of value) or she remains very dependent, needing validation from others in order to act.

Like middle sisters, twins tend to be good mediators and more so than any other sister position, seek harmony. They are often used to having someone who knows what they’re thinking without having to state it and try to replicate that in friend and love relationships, frequently being disappointed that others don’t just ‘get’ them without a lot of explanation.

II. You Were Always Mom’s Favorite!: Sisters in Conversation Throughout Their Lives (2009), by Deborah Tannen

Linguist Tannen’s research involved more than 100 women.

From a New York Times essay by Tannen:

…(T)he key to why having sisters makes people happier — men as well as women — may lie not in the kind of talk they exchange but in the fact of talk. If men, like women, talk more often to their sisters than to their brothers, that could explain why sisters make them happier.

Two quotes from a book excerpt on NPR:

The word “sister” evokes an ideal of connection and support, like the friendships that made Rebecca Wells’s Divine Secrets of the Ya-Ya Sisterhood and Ann Brashares’s The Sisterhood of the Traveling Pants into bestselling novels and successful films…Part of the reason these books and movies were so popular is that we all yearn to belong to a group with a bond like that. As one woman put it, “Friends are the sisters we were meant to have.”