Ashley Pro lived with her sister Dennice for 28 years. They moved in together after leaving their childhood home, and if Pro hadn’t been transferred to a different city for work, the sisters would likely still be roommates. Pro, a 29-year-old director for an after-school program in Rancho Cucamonga, California, initially worried that any distance would drive a wedge in their relationship. She’d never known a life without the daily presence of Dennice, who is only a year older.

Since their mom worked long hours to make ends meet, Dennice took on a maternal role with her little sister, even picking up extra jobs in college to pay for Ashley’s high school extracurriculars. Although Ashley and Dennice are close to their three older siblings, this period of reliance bonded them.

Even now that they’re living separately, about a 30-minute car ride apart, Ashley says the sisters are as close as ever. They talk on the phone regularly and spend weekends at each other’s places. “It’s something we envisioned,” Pro says. “That was our goal growing up, so we made sure to keep that relationship strong.”

In what may be obvious to those who have them, siblings stand to be one of the most enduring relationships of a person’s life. They’re your first roommates, your first playmates, maybe your first babysitter or charge, and probably your first fight. They’re your social guinea pigs, the first draft of nearly every interpersonal interaction. Siblings, including half-, step-, and adoptive brothers and sisters, are thrust upon you. But as you age, maintaining those relationships is voluntary.

They’re your first roommates, your first playmates, maybe your first babysitter or charge, and probably your first fight.

As siblings progress through life, these once-obligatory relationships can transition from roommate to friend or even best friend. In interviews for their 2015 book Adult Sibling Relationships, authors Geoffrey L. Greif and Michael E. Woolley found 64 percent of respondents said they were good friends with a sibling; 45 percent considered a sibling one of their best friends. But the sibling relationship can also be more fraught. Greif and Woolley found that 62 percent had mixed feelings about their siblings, feeling neither wholly lovey-dovey nor completely cold (interestingly enough, even those who are close to their siblings can have such mixed feelings).

However you feel about your siblings, it’s clear these relationships have a profound impact on well-being. Into adulthood, those who perceive parental favoritism or sibling conflict are more likely to have symptoms of depression, anxiety, hostility, and loneliness. Adult sibling relationships hold just as much weight as a person’s relationship with their mother or spouse.

But what if, for one reason or another, your relationship with a sibling is cordial at best? What if it feels like a relationship you never would have maintained if not for being connected by blood or family ties? “It’s not a bad thing that you don’t have a super close relationship with a sibling,” says Katherine Jewsbury Conger, a professor emerita of human development and........

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