It’s a tall order to expect two adults to maintain the same relationship they had in their childhoods.

Genetics and similar childhoods aside, it’s wishful thinking to expect siblings to take up as much space in each other’s lives as they did growing up. It’s also naive to assume that distancing yourself from a sibling is inherently a bad thing, said Kiaundra Jackson, a marriage and family therapist in Los Angeles, California.

“I think we are delusional if we expect sibling relationships to be the same from childhood and even more delusional if we misconstrue sibling differences as sibling rivalry,” she said. “These two are humans. They change, evolve, grow and want different things during different phases of their lives.”

Stripped of their titles, the Cambridges and Sussexes’ supposed rift is a very simple story. It’s easy to concoct a non-royal version of the scenario: Imagine two siblings set to inherit a family farm (as co-owners, if their parents get their wish). While one sibling might be eager to run the family business (or is simply willing to do it), the other might harbor dreams of law school and life in the city. Neither choice is more valid, just altogether different.

When the farmer and lawyer are of marrying age, though, things get doubly complicated: Toss in two spouses with entirely different worldviews and expectations on family life and the family business (Should profits from the farm be divvied up differently? Why not just sell it already? Why isn’t your brother the big-shot lawyer helping out more?) and the strain increases. Factor in disparities in financial success and differing political views and it’s nearly impossible for an estrangement not to happen.

If cooler heads can prevail in the family, though, the decision to set boundaries, respect individual choices and allow space is in the best interest of most parties, Duffy said.

“At times, to be honest, I find this to be a somewhat healthy boundary. I never think it’s a great idea to exclude family entirely from your life but sometimes, in toxic family situations, strong and powerful boundaries can help preserve relationships,” he said.

Childhood rivalries can seep into adult relationships.

In many cases, a lingering childhood rivalry sets the rift in motion, said Jeanne Safer, a psychoanalyst specializing in sibling issues and the author of “The Normal One: Life with a Difficult or Damaged Sibling.”

Maybe growing up, the parents played favorites or pitted the kids against each other when report cards came in or with extracurricular activities. Or maybe, as in Harry and William’s case, a constitutional monarchy and natural order of succession lent an air of competition to their childhoods: While William has always been groomed to be the future king of England, Harry has dropped further and further down the line of succession with each new Cambridge kid. (Current position? No. 6, making him the spare-spare-spare-spare-spare-spare to the heir. Why wouldn’t he want to pave his own path?)

It’s the hierarchal aspect of this story which makes it such an interesting (and, in many ways, relatable) case study in sibling dynamics, Safer said.

“William and Harry really bring rivalries into focus,” she said. “Many siblings never recover from the bitterness of unequal distributions, whether it’s the crown or money. Even if your brother or sister is not going to be the king or queen of England, there can be overt or covert parental preference, envy and misunderstandings that are never discussed.”