One of the raps on single people is that they create alienation and anomie. A nation that would be connected, family-oriented, and community-minded if all those singles were married instead becomes a nation of isolates because of all the people living single.
I've often mentioned a set of studies that suggests quite the opposite. People who have always been single are more likely than currently married people (especially) or previously married people (to a lesser extent) to help, encourage, visit, and communicate with friends, neighbors, siblings, and parents. That's what Naomi Gerstel and Natalia Sarkisian found in their paper titled "Marriage: the good, the bad, and the greedy." The data relevant to sibling relationships came from the 1992-1994 National Survey of Families and Households. That's a nationally representative sample, so that's the good part. However, the analyses were from just one point in time. So, it was not possible to know for sure that the married people were less often in touch with their siblings because they got married or that the previously married people tended to maintain more contact with their sibs than currently married people did because they got unmarried.
As I continue to research my chapter on the place of family in the lives of people who are single with no children, I've been trying to learn more about siblings. I found a report of an earlier wave of that National Survey of Families and Households, from 1987-1988. Once again, results showed that people who had a spouse or partner were less likely to visit or call or write to their siblings than were those who were single. Again, though, the data were from just one point in time.
Happily, sociologist Lynn White has studied the changes in contact with siblings between the 1978-1988 survey and the 1992-1994 data collection. Following the same people over time, did those who got married have less contact with their sibs than they did before, and did people who got divorced have more contact? The answer to both questions is yes (from Table 3, if you can access the article).
To answer the question in the title of this post, the people who provide the glue to sibling relationships in adult life are those who are single. People who have always been single are especially good at this, but the previously married also do better than the currently married.
You can probably guess the matrimaniacal explanation that is sometimes offered for findings like this (it also comes up in discussions of singles and their pets) - single people are just "compensating" for not having a spouse. By that way of thinking, single people don't really value their siblings (or their parents or the friends or neighbors they stay in touch with more than currently married people do) - those other people are just consolation prizes.
Gerstel and Sarkisian's "greedy marriage" explanation is different. Discussing married people's weaker ties to siblings, parents, neighbors, and friends, the scholars note that marriage can "demand a kind of intense emotional involvement that by itself detracts from collective life...Finding a soul mate means turning inward - pushing aside other relationships."
They are quick to note, though, that greediness is not intrinsic to marriage. Instead, it seems to be a by-product of the intensive way that marriage and coupling is practiced in contemporary American society. I would also add that there are differences from one married person to another in the degree to which they regard their partner as the near-exclusive focus of their adult social life.
Compared to spouses and romantic partners, adult siblings get little attention from academics. By their neglect, scholars are enabling the clueless question often posed to people known to have siblings: Do you have a family?