No matter how hard I tried, I could never manage to add a sister to my life. And try I did: I asked for a sister for my birthday year after year. So did my daughter, but she got lucky. Because I remarried, I managed to snare two wonderful sisters for my daughter in the wedding package, and all three women are delighted. They giggle, talk about me behind my back, and love each other thoroughly. Giving my daughter two older sisters has been one of my greatest gifts to her.

Since the relationship is not one I experience daily, I was surprised when my cousin Claire, herself the younger sister of two daughters, mentioned that she had gotten a wonderful spoonful of wisdom before the second of her two daughters was born. A friend had told her: “You are the most important person to your older daughter. But your older daughter will be the single most important person in your younger daughter’s life. Don’t ever overlook that."

This statement might have been hard for me to absorb, had I not seen the energy between older granddaughter Ava, nearly five, and tiny, robust sister Sara, not yet two. They call each other “Sissy” in that special language that sisters develop, and they operate with dynamics I usually associate with well-functioning marital couples. Ava not only watches out for Sara at every turn, she has the competitiveness with her younger sister usually reserved for equally qualified applicants for the same executive position: The beautiful dark haired, blue eyed Ava is subtle in her attempts to jostle Sara, with huge hazel eyes peeking from blond bangs in her round face, from her honorary place as “just about the cutest thing on two legs.”

Sara walks like a lumber jack in her well-filled leggings and tummy hugging tops.Throwing out her tummy, shoulder back, all 30 inches of her marches independently forward so a saunter with her on Stone Harbor’s 96th Street brings constant smiles from well-dressed ladies strolling the shops. In public, Ava is the perfect older sister: polite and protective, she helps “Sissy” reach high items in shops, and reminds me if I forgot to buckle her seat belt. Ava is so lovingly protective that Sara has her own bodyguard. Sara and Ava are so devoted to one another that, on a recent trip I took with Ava, she told me that she missed “Sissy” before she told me that she missed Mommy and Daddy.

However, sisters share complex and layered relationships, and a few hours with Ava and Sara tells the rest of the tale. In private, when the two need to share one beach wagon or one package of cookies, the rivalry pops out. Sara’s first words included, not only “Dada,” but “MINE!,” uttered with all the assertiveness in her tiny body. There is no hitting or yelling: other than the occasional two-year-old outburst, the two work out differences through wining, asking for an adult to judge who plays with the only red wagon, and who gets attention.

As the first grandchild of four child-centered grandparents, Ava has had quite an adjustment. But, for her hard work, Ava reaps adoration and wide-eyed heroine worship: If Ava uses the big jungle gym, so must “Sissy,” who pushes to be like Ava by fearlessly taking on heights. If Ava wears a bow in her long silky brown hair, Sara insists on gathering her sandy wisps into a bow-topped arrangement, creating the appearance of a decorated apple. And, if adoration weren’t sufficient, Ava is also undergoing leadership training fit for executive tasks: “Sissy, you need to wear a hat in the sun,” “Sissy, you need to go for your nap.”

Cambridge University scholar Terri Apter interviewed 76 sisters from 37 families and has captured well the complexity of sisters, who customarily combine deep love with rivalry, just as we see with Sara and Ava. Theories of human development report that female relationships often combine a mix of emotions: loyalty and disinterest, love and hate, envy and admiration. Sisters’ relationships can be as influential in determining in adult personality as the parental relationship: sisters both need to differentiate themselves from one another and cherish the deep protectiveness they feel for the same person. For this reason, a sister’s death can be devastating, and adult female friendships often capture the dynamics formed in early sister relationships.

To consider: Cousin Claire’s wisdom is worth contemplating. Do you agree that the most important single influence to a younger sister is her older sister? And how might this awareness impact your life?

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