This summer, all it took for Jeremy MacKechnie to pull the trigger on a trip with his husband, brother and sister-in-law — something the two couples had been loosely considering, with little headway — were $7 upgrades on flights to Cancún that seemed too good to be true.

“I decided to just book first and ask questions later,” said Mr. MacKechnie, whose daughter was then 18 months old. His brother and sister-in-law hadn’t taken a break since their sons, then 2 and 7, were born.

“Now that we have kids, we don’t get that much time to just hang out as adults,” said Mr. MacKechnie, 33, who works at a tech start-up in New York City. “It was the best vacation I ever booked.”

Whereas Mr. MacKechnie and Marissa Talbot are both the designated planners, other siblings are more likely to share that task.

Priya Gursahaney, 37, an obstetrician-gynecologist in Cincinnati, and her younger brother, Vivek, 33, who lives in Columbus, Ohio, both have “strong personalities” — her words — and collaboratively craft vacations that suit their mutual love of culture-rich cities and active travel.

Together they have attended a string quartet concert at the Municipal House, a famed building in Prague, and visited La Recoleta Cemetery in Buenos Aires. But what the Gursahaneys value most are the simpler moments; say, sharing a packed sandwich at Laguna de los Tres, a lagoon in Patagonia, while gazing upon the staggering Monte Fitz Roy.

“It’s a nice thing to have, especially as we get older. I don’t have a family of my own yet, so all the more reason to share that closeness with my brother,” Dr. Gursahaney, 37, said.