Manhattan private schools used to be like hand-me-downs or fancy china: At the key moment, parents would pass the school down to their sons and daughters; later, younger siblings would get their turn.

But this year, as ever-larger families flood Manhattan, sending ever-greater numbers of applications to a basically stagnant number of private schools, the family way is eroding. Many schools give no leg up to so-called legacy applicants, the children of alumni. Siblings, though they often do get a boost, are increasingly being encouraged not to count on their brothers’ and sisters’ schools.

“It’s not a joke anymore,” the founder of Manhattan Private School Advisors, Amanda Uhry, said. “If you have a sibling, you’d be crazy not to apply to a number of schools. You’d be nuts not to do that.”

At the Calhoun School on the Upper West Side, several classes have recently received more applicants with brothers or sisters already at the school than there are spots, forcing at least a few siblings to be pushed out, the school’s head, Steven Nelson, said.

Columbia Grammar, also on the West Side, has eliminated preferences for the children of alumni, its admissions director, Simone Hristidis, said.

And the Horace Mann School in the Riverdale section of the Bronx has eliminated both kinds of preferences: for siblings and for legacy children, several sources said.

The school did not respond to requests for comment yesterday.

An official at another uptown private school who asked not to be named said discussions over whether to eliminate preferences for siblings are ongoing.

The pattern of families choosing to stay in Manhattan rather than moving to the suburbs is one reason for the tightened policies. So too are newly constructed luxury buildings promising large, child-friendly apartments — the Manhattan House on the Upper East Side, for instance, boasts an indoor playhouse conceived by the designers of the Children’s Museum of Manhattan.

“When I started in my evaluation business, in the mid-80s, the new development focus was on entry-level apartments – studios and one-bedrooms,” Jonathan Miller, the executive vice president of a real estate data firm, Radar Logic, said. “That’s clearly changed.”

Admissions experts said another factor in the sibling crunch is that, in addition to more families, there are simply more siblings in the city as the result of larger families.

“Four is the new three, three is the new two, two is the new one,” Ms. Uhry said.

A demographics expert who chairs the sociology department at Queens College, Andrew Beveridge, described the change as part of the new baby boom among the wealthiest Manhattanites. “The kid becomes like another item of conspicuous consumption,” Mr. Beveridge said.

For school admissions staffs, quadruple the siblings means quadruple the headaches. Not only do the extra bodies mean more applications to sort through; they also create tough choices about how to pick their class, such as whether to admit a year full of mostly male siblings and sacrifice gender balance— or whether, alternatively, to force it but reject some of the brothers.

“This is a big issue for lots of schools, and there are schools that are taking away their sibling policy,” Ms. Hristidis at Columbia Grammar said.

Ms. Hristidis said her school has managed to preserve its preferences so far by eliminating boosts for the children of alumni — and shutting out a large number of applicants who are not siblings in the meantime. This year, for instance, she has so many male siblings applying to next year’s kindergarten class that she predicts few “outside” boys will be admitted.

“I’m not going to say a number, because I don’t want to set off any more panic out there than there already is,” she said. “But it’s going to be very few.”

At Calhoun, Mr. Nelson said his admissions staff sometimes makes the opposite choice. For that reason, he said, the school makes sure to encourage parents to apply to more than just Calhoun when their second, third, and fourth children reach kindergarten age.

“We love having what other people call legacies — families who have had kids in the school before, or children of Calhoun graduates — so we have a very clear intention to give them preference,” Mr. Nelson said. “But, there can’t be a guarantee.”

Some said that creates headaches for families, who with children at multiple schools are forced to juggle extra duties, such as manning safety patrol; attending teacher conferences; and giving money.

Ms. Uhry, however, said she supports the change.

“Just because your sister is bright enough to get into a school doesn’t really mean you are,” she said. “It kind of throws Clorox in the gene pool if you take kids who are substandard.”

She said parents’ desires to send their children to the same school is selfish; a child should go to the school he is best suited to, not the school best suited to his sister. “It is better to be the top kid at a second-tier school than to be the bottom kid at a top tier school. I can guarantee that to you as a parent: You will raise a happier child,” she said.