The week after Thanksgiving is a time for complaining about relatives. Everyone seems to come back from vacation with a story. A friend tells the one about how her brother-in-law showed up on her doorstep Thanksgiving morning with two large, untrained dogs, despite her repeated entreaties that he find a place to board them—she has four kids and two cats. A fight ensued, and her mother- and father-in-law wanted everyone to make peace. The details are not important—the upshot is always the same. Parents want to know why their adult sons and daughters can’t get along. They rack their brains trying to figure out where they went wrong.

But in principle there is no reason we would be any more likely to be friends with our siblings than any other young adult with a similar background. You can’t pick your family. And it seems that sibling relationships are a mixed bag from early on. According to researchers at Brigham Young University, brothers and sisters can positively or negatively affect the way a person interacts with others. “Sibling affection was positively associated with adolescents’ sympathy and prosocial behavior.” Meanwhile, the authors also found that for boys, “Sibling hostility was positively associated with adolescents’ depression and externalizing behavior.”

But what happens when those siblings grow up? Do the people who experienced sibling affection as youngsters tend to have better relationships with siblings as adults? And what of the reverse? Do we know that poor relationships with siblings as children lead to estrangement for adults?

There’s not a great deal of research out there on the effects of sibling relationships, and there is even less on how they affect people as adults. Which is why I was particularly interested in a study released last year by three Ohio State University sociologists, finding that having more siblings reduces the likelihood of divorce. Now it wasn’t a great reduction—controlling for background factors, each additional sibling, up to about seven, reduces the risk only 2%—but still, what is it about having more brothers and sisters that makes one’s marriage more stable?

Perhaps these people are better adjusted because they experienced sibling affection as children. Or perhaps in adulthood their siblings are providing some emotional support. As Robin Marantz Henig points out on a blog post at NPR’s website, “We’re tethered to our brothers and sisters as adults far longer than we are as children; our sibling relationships, in fact, are the longest-lasting family ties we have.” Two-thirds of people in one 1992 study reported that a brother or sister was also their best friend.

Either way, it’s useful to note because we have fewer and fewer siblings these days. Declining fertility rates mean we will get less sibling affection and less sibling support—as children and adults.

It is perfectly understandable that parents of adult children will want them to get along with each other in adulthood. (“Who will you have left after I’m gone?”) But is there anything they can do to foster better relationships among children?

One 2009 study suggests that parents may be able to help their kids maintain good relationships by avoiding favoritism: Siblings who said that their mother didn’t show much favoritism when they were growing up had better relationships as adults than siblings who reported favoritism. And among those who said that their parents played favorites, it didn’t matter if they were the favored one or not—either way they tended to have lower perceptions of their current relationships with their siblings.

I think it is awfully hard to sort out these matters as adults. People’s recollections of their childhoods are always going to be skewed. It’s also possible that siblings who didn’t get along as children felt that their parents were taking sides in their conflicts and those children also grew up into adults who didn’t get along well either. No doubt it’s better for family harmony if parents treat their children equitably. But some people are always going to bring unwelcome dogs to Thanksgiving dinner.

Naomi Schaefer Riley is a columnist for the New York Post and author of  ’Til Faith Do Us Part: How Interfaith Marriage Is Transforming America.