Both are wisp thin. Peter’s deadpan, detached demeanor contrasts with Harry’s livelier, impish quality, a witty rejoinder ever ready. The Brant brood is rounded out by a younger sister and an older brother in college, plus five half-siblings from Mr. Brant’s previous marriage to Sandra Brant, but Peter and Harry are the nearest in age and have always been close.

Despite their youth, the boys are omnipresent on the social scene and staples of Patrick McMullan party photographs. Their every move is tracked on assorted fashion blogs. Earlier this year, Refinery29 touted, “Meet NYC’s Most Beautiful Teenage Brothers.” Their comings and goings are chronicled on the New York magazine blog The Cut.

“Everybody loves celebrity children,” said Stephanie Trong, the editorial director of The Cut. “But perhaps the biggest appeal is that these guys live in the lap of luxury and they’re extremely open about their exploits. How many teens go to couture shows or fashion parties, much less document them on their joint Twitter feed, in such a hilarious, uncensored way?”

The Brants have almost 70,000 Twitter followers, a fraction of whom appear to be their age.

“Most of my tweets happen between 1 and 5 in the morning,” Harry said. “I’m a night owl, and random thoughts pop into my head. I’ll be watching ‘Mommie Dearest,’ and I’ll be like, ‘Oh, my God, Joan Crawford is amazing.’ ”