What Makes Sisterhood So Special

Of all possible family relationships, those with siblings have the potential to be not only the longest but the strongest. You are united with your brother or sister in a union that can persist for decades, far longer than any with a friend or romantic partner. If you have a sibling, think for a moment about the early experiences that shaped who each of you are today. If you don’t have a sibling, ponder the relationship that you have or had with close cousins or family friends.

Because siblings first “meet” when they are young, their developmental trajectories become interlaced with other key early milestones. Siblings are together throughout all the major life events of childhood and adolescence, from losing a first tooth to having a first romantic partner. They also can form an alliance to protect each other from a variety of other key figures including parents, teachers, or the other neighborhood children.

Despite the obvious importance that siblings have and can have for each other, research in this area is virtually nonexistent. Fortunately, a new study taking a longitudinal (long-term) approach provides valuable new insights into this fascinating area of family dynamics.

Defining the Key Factors in Sibling Relationships

According to Pennsylvania State University’s Susan McHale and colleagues (2024), even with some growing interest in this field, there are few that follow sibling patterns into adulthood. The Penn State researchers took advantage of an existing study of 201 families begun in 1995-96 which continued for 18 years across nine times of testing. At the study’s outset, the average age of the firstborns was 12 years, about 18 months older than the secondborns. The sibling pairs were divided almost equally by pairing of the sex of the child. By the end of the study, there were 61 families left, and the firstborns averaged 28.7 years, and the secondborns 26. At each test point, their parents participated as well.

If you’ve been a sibling yourself, you can probably guess what the key variables of interest were, namely intimacy and conflict. In general, prior research supports the commonsense idea that conflict tends to decrease and intimacy to increase as siblings move beyond their direct involvement with each other in the nuclear family and out into the world. The research question guiding the study was whether these patterns could be predicted by sibling sex pairings and family position. The authors also chose to include measures of expressivity to find out how this factor, typically stronger in women, might influence the unfolding of relationship patterns over time.

The Trajectory of Sibling Relationships into Adulthood

McHale and her coauthor team tackled these complex issues by using statistical methods that allowed them to chart changes within family constellations on the variables of conflict and intimacy. They also were able to stack the data so that they could construct an “accelerated longitudinal design,” meaning that instead of only having nine years at their disposal, they could statistically construct a nearly 25-year time span (from ages 7 to 31).

If you would like to see how you would score on their main sibling variables, try rating your relationship with your own sibling or another close family member if you’re an only child. An example of an intimacy question was “How much do you share your inner feelings or secrets with (sibling)?” For conflict, ask yourself, “How often do you feel mad or angry at your sister/brother?” Each of these would be rated on a scale of 1 (not at all) to 5 (very much). Participants also rated the warmth and conflict with their children with statements such as “I understand my child’s problems and worries” (warmth) and, for conflict, in areas including appearance, social life, health, money, behavior/personality, and sibling relationships. Finally, items measuring expressivity asked participants to rate such qualities as sensitivity and kindness.

Given the many possible combinations of sibling dyads along with parental measures, the analyses required a considerable degree of honing. Eventually, using sophisticated longitudinal analytical methods, the authors were able to chart not only patterns of change, but the family structural variables that influenced those patterns.

Overall, the findings showed, as the authors predicted, increases in intimacy and decreases in conflict, but not in a straight linear fashion. Intimacy declined slightly up through early adolescence and then continued to increase until it leveled off in early adulthood. Conflict showed precisely the opposite pattern. The adolescence-early adulthood period, then, presented a critical turning point.

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Influencing the nature of these changes over time, however, was the sex of the sibling dyad. There was a “curvier” pattern for sisters than brothers, especially in the mid-20s. Additionally, sister-sister pairings were more positive, supporting prior findings that “sisters may serve as the glue that holds a sibling dyad together, at least until young adulthood." After that, a pattern of “selective optimization” sets in as even the closest of sisters find that they have to divide their emotional attention among their own families as well as toward their work.

With respect to emotional expressivity, the findings further accentuated the role of sister-siter pairings, who generally had more positive relationships. In the words of the authors, the findings are consistent with previous research showing that “happiness is a feminine partner."

The warmth that parents expressed toward their siblings further influenced the dynamics of conflict and intimacy. When parents showed greater warmth toward their children, the siblings got along better, too, supporting “the enduring importance of close relationships with mothers." The role of the father was somewhat different, in that having a less warm father seemed related to greater intimacy between siblings, as if they need to form stronger bonds when the family relationship climate becomes strained.

Getting the Most Out of Your Sibling Relationships

As you can see from the findings, time may erode some of the closeness between even the closest of siblings, but not due to lack of love or warmth. Life can get in the way of even the best sister-sister pairing. You may not be the best of friends with your sister now (if you are a sister) but chances are that, over time, the bottom line will not erode.

Regarding the “feminine partner” effect of warmth, the authors turn to socialization as an explanation rather than anything intrinsic about biological sex. “Gender roles and ideologies” appear to be just as likely a possibility as inheritance.

The obvious question you may be asking is whether, if you have no sister, your life is fated to lack a key emotional dimension. The findings suggest that the unique quality of sister-sister pairing may be worth trying to achieve with other people in your life. You may be too busy now, but as some of those obligations that preoccupy young adults lessen, it is possible that sisters can return to their previous levels of intimacy.

What if you have a sibling from whom you’ve become estranged? Looking at these results may help you understand how this situation evolved, and maybe why it may be worth giving intimacy another try.

To sum up, having a sister if you’re a sister seems to offer unique emotional rewards beyond the years of childhood. Finding intimacy with people who are not your romantic partner may provide you with these rewards as your life trajectory unfolds.