I’ve always liked Cheri Register’s definition of “siblings” in her book, Are Those Kids Yours? Asked whether her children were “really” brother and sister, she’d respond, “Well, they fight in the back seat.” That’s a remarkably cogent way to deal with a question that is a) none of anyone’s business outside the family and b) loaded with hidden meaning.

We’ve all had run-ins with prying individuals, but the second aspect of the “really” question bears closer inspection. Unfortunately, the erroneous belief that parent-child bonds in adoptive families are not as intense as in biologic families still sticks.

So, when someone asks, “Are they really brother and sister,” parents may wonder whether the assumption driving the question is that non-biologically related brothers and sisters feel less connected, have less (or more) sibling rivalry, or have less (or more) influence on one another.

Register’s comment brilliantly suggests that sibling relationships develop because children grow up together. At the same time, her comment is a reality check for parents who think that neither aggression nor jealousy would arise if their children were biologically related.

A shared history

Sibling relationships, experts tell us, are remarkably complex and deeply influential. While even children close in age do not have identical experiences growing up, they share the same upbringing. They know their family history better than anyone else, and they carry that history with them into every relationship they develop, long after family members have moved apart, long after parents have died.

Having someone who knows them so intimately can be even more important to adoptees when they do not have strong connections to their extended adoptive families, have little or no contact with birth relatives, or have little or no knowledge of their heritage or genetic background.

And even when adoptees maintain sibling relationships through open adoptions, or connect with birth siblings later in life, it is their adoptive siblings who share first-hand knowledge of what it was like for each other growing up and can share memories as adults.

Identity development

Perhaps the most important function siblings serve is to aid in the development of personal identity. Identity is a complex picture composed of gender, race, sexual orientation, religion, abilities, personality, and other attributes.

Identity formation begins in early childhood, but it becomes a primary task during adolescence. Teens compare themselves to the people they are most like – parents and siblings – to discover what makes them unique.

In adoptive families, similarities are psychological, derived from a shared environment rather than common DNA. In addition, adopted teens compare themselves to what they know or imagine their birth parents and siblings to be like, making the process more complicated, though not necessarily problematic.

While we each have a variety of identities – in some contexts I think of myself as a writer; in others, as a wife or mother – we each have a “core” identity. Stephen Banks and Michael Kahn, authors of The Sibling Bond, describe this as the “truest, strongest, deepest part of the self that is essentially unchanging and is maintained throughout life.”

Children are more likely to let siblings glimpse this core identity, say Banks and Kahn, than to let their parents. The flip side of this trust is the power siblings have to reveal each other if they are projecting false personae to the world. Likewise, if a teen is trying to project an identity that “fits in” with his adoptive family, or is trying to reflect what he thinks he would be, if he had been raised in his birth family, his sibling is the person most likely to know it.

So now it becomes a little clearer why a sibling sometimes doesn’t want her brother breathing on her. It’s about the risk that comes from being close.

Our role as parents

As important as sibling relationships are, parents are not responsible for making or keeping them. Parents are responsible for nurturing each individual child – meeting the unique needs and loving each one unconditionally. Each child is then responsible for developing his or her own relationships — both within the family and outside of it – from this emotional base.

In adoptive families, the primary reason to adopt a second child should be that the parents want to nurture another child, not to give their first child a sibling, a playmate, or someone else in the family with the same racial or ethnic heritage. Nor should parents deprive one child of the benefits of an open adoption because it might cause another child to feel the absence of his birth family more acutely. Instead of ignoring differences, we must allow our children to express their feelings about them.

Parents aren’t responsible for equalizing differences between children. Adele Faber, author of Siblings Without Rivalry, cautions parents against trying to treat their children equally so as not to show favoritism.

Instead, she says, parents should treat children uniquely. Siblings generally know each other so well that parents risk losing credibility when they pretend not to see one child’s gifts. This means that family resources or attention may sometimes be disproportionately focused on one child, but as long as the other child’s needs are not neglected, it may be what’s necessary.

When it comes to fostering a child’s gifts, parents must stay attuned to the child’s needs or wants, rather than imposing their own expectations. Forcing children into roles based on parental assessment of their gifts is not necessarily the same thing as nurturing their interests.

For example, even though Jenny doesn’t have her sister’s natural talent at the piano, it’s more important to her to express herself musically than to cultivate her superior athletic ability. Avoid restricting children’s interests or in any way implying that they aren’t valuable. What “compensates” for any inequality is unconditional love we give each child — love that is unattached to abilities or accomplishments.

Lasting bonds

Parents help their children and teens develop their core identities when they see each child’s unique gifts and characteristics, as well as the ways in which their children are similar. Parents have to reflect back to their children their truest selves.

Some factors that impact sibling relationships are beyond parental control – temperament, personality, abilities. In addition, adoptive parents cannot always ensure that children are close enough in age to share time, space, schools, and family history.

What parents can do is provide each child with unconditional love, as well as help each develop as an individual without imposing expectations or creating distinct roles.

This can be a daunting task. Continually checking your motives will help. If parents make decisions “so that Ethel doesn’t feel…” or “so that Gerhard knows…,” chances are, they’re trying to control how their child interprets an event rather than acting from a sense of what their child needs.

And, in the end, parents’ own experiences growing up with (or without) siblings may affect their expectations for their children’s relationship. With some honest reflection, most of us can remember what it was like as a child, when a sibling breathed on us – and compare that to how close we became once we left home.