“Siblings by chance, best friends by choice,” the cliché goes. But ask real people about their own family dynamics, and their reactions will probably vary. Some siblings are inseparable, talking daily and sharing even the most TMI dating updates. Others, however, have a bond that’s just cordial or, in some instances, completely estranged—as is allegedly the case with celebrities Haylie and Hilary Duff.
In her new album Luck…or Something, Hilary seemingly addressed their long-rumored rift in the song, “We Don’t Talk,” which includes lyrics like, “Cause we come from the same home, the same blood,” and “People ask me how you’re doing, I wanna say amazing / But the truth is that I don’t know.” While she doesn’t name the subject directly, she referred to this distance as “the most lonely part of my life” in a Rolling Stone interview published Thursday.
But even without a dramatic fallout, the reality for many families is that “it’s normal for brothers and sisters to be more like friendly acquaintances who happen to share overlapping lives,” Erin Runt, LMFT, a Chicago-based licensed therapist, tells SELF. “There’s this idea that how frequently you’re in contact represents how emotionally close you are.” Realistically, however, that isn’t the case for many people (despite what family sitcoms or your friends’ picture-perfect Instagram stories might suggest)—and it’s nothing to feel guilty about either.
So what makes some siblings best friends while others just coexist? Aside from a major conflict, here are a few factors to consider, according to family therapists.
1. Parental involvement
In some households or cultures, parents are intentional about raising their kids to be close: to look out for and take care of each other, to see each other as built-in allies from a young age. Maybe your mom pushed you to include your younger sister in all of your playdates, or your older brother was the one who taught you how to swim, ride a bike, and drive. According to Runt, early experiences of support and collaboration can quietly shape how siblings relate to each other as adults.
2. Favoritism in the family
Just as positive experiences can bring siblings closer, negative ones can drive them apart. “Favoritism is one of the many things parents do, even if it’s unintentional,” Karen Gail Lewis, MSW, EdD, Washington, DC–based therapist and author of Sibling Therapy: The Ghosts that Haunt Your Client’s Love and Work, tells SELF. It doesn’t matter whether you’re older or younger: The one who didn’t get “special treatment”—whether that meant less leniency, more criticism, or constant comparisons—might hold onto resentment, which can make it harder to build a positive, trusted relationship over time.
3. Shared life experiences
Even if siblings didn’t get along as kids or teens, Runt says they’re more likely to bond later if they hit the same milestones. “Maybe they both started having children or got married around a similar time,” she says—changes that may naturally lead them to swap advice or lean on each other for help. Ultimately, these are experiences that can create fresh common ground and make the bond feel more organic, not forced.
4. The size of an age gap
While people love to speculate the “ideal” gap between siblings, “there’s no clear pattern that predicts closeness in terms of age or sex or gender,” Gail Lewis says. In some instances, being six or more years apart can make it harder to relate to each other on a peer level, whereas growing up on a similar timeline can create a more friend-like connection.