Writer and  Georgetown University Linguistics professor Deborah Tannen wrote a terrific book about the complex relationships between sisters, called You Were Always Mom's Favorite that might really help you understand the relationship better if you have a sister, are a sister, or have daughters of your own (Ballantine Books, 2010). Sisters have a life-long conversation that can reveal much about each of them.

As it turns out, girls often define themselves by the differences between themselves and their sister or sisters. There is often a lifelong tug of war between competition and closeness. Sisters compete for parents' time, attention, and financial support. Each sister tries to carve out her own identity within the family. If an older sister is seen by the family as the "smart" one, a younger sister might seek to differentiate herself by assuming a different identity.

Birth order plays a role in the relationship between sisters. Tannen found this even between twin sisters, where there is still a focus on who arrived on the scene first. Older sisters may be called on to help with the care, supervision, homework and responsibility for a younger sister by parents, and later be resented by a younger sister for treating them as a parent treats a child. Younger sisters may be babied and protected. Adult sisters sometimes have trouble renegotiating childhood roles. For example, ideally it would be best for an older sister to build a mutually respectful relationship, and not try to know more or be the boss all their lives.

As Tannen and her researchers studied conversations between sisters, they foundsomething she labeled "sister speak" where sisters can develop their own unique conversational flow and share the telling of stories. Because sisters usually share a common history, they can have conversational shorthand that they understand but others might not completely get.

Alignment in the family is important. If one sister is seen as "closer" to mom or dad, it can cause other sisters to feel left out.  Asa structural family therapist, I often want parents to be more aware of having as many good kids as they have children, and not joining in an alliance with one child, or playing favorites.

As adults, resentments can be exacerbated by one sister living closer to aging parents and assuming a larger or solo role in managing parents' increasing needs. Some adult sisters opt out and leave caring for aging parents, by default, to another sister. Some mothers add to sister conflict by over-praising one sister or her children.

Your sister might  represent the path you did not take in life. Acceptance, tolerance, and mutual respect can help soothe differences. So can lowering your expectations about your sister, and adding in other close female friends.

Sisters can be such different things to different women. It's wonderful when they are close emotionally, caring, supportive, and interested. Many women feel the loss of what they would have longed for if their sister is competitive, cold, disinterested or antagonistic. I have found women who feel sadness that their sister order got messed up.

Whether your sister is a strong ally or a stiff competitor, chances are she's one of the few people on the planet who knows your whole life story. The relationship between sisters, or the lack of it, helps to define us as women and as individuals.