But the company was upended when Charles became engulfed in a nasty family feud over how the business’s proceeds were to be distributed. The fight, which played out in a federal courthouse in Newark, resulted in a plea deal for Charles, who in 2005 was sentenced to two years in prison for tax evasion, witness tampering and making illegal campaign donations. The family infighting was so bitter that, at one point, Charles hired a prostitute to seduce his brother-in-law, videotaped the encounter and sent the footage to his sister.

Jared, 23 at the time of his father’s conviction, had recently graduated from Harvard. He was studying for an M.B.A. and law degree at New York University in 2006 when he bought The New York Observer, at the time an influential weekly newspaper known for its coverage of the city’s elite and high-end real estate.

It is unclear exactly when he assumed control of the family business. The company now says he became chief executive in 2008, but contemporaneous news accounts rarely describe him that way until 2012. Nevertheless, Mr. Kushner quickly became the company’s public face as it expanded across the Hudson River into Manhattan, much as Mr. Trump had left Queens for the big city decades before.

Charles Kushner was released from federal custody in August 2006. He immediately resumed a significant role in the business and remains heavily involved today. Still, it was with Jared as headliner that the company soon made its biggest play ever: $1.8 billion for the skyscraper at 666 Fifth Avenue that would remain at the center of its story to this day. It was the highest price ever paid for a single office building in the United States — and more than three times what its seller had paid six years earlier.

Around this time, Mr. Kushner met the woman he would marry: Ivanka Trump. “J-Vanka,” the headlines blared, as the New York tabloids celebrated a match made in real estate heaven.

Everything was looking up, until suddenly it wasn’t. Within a year after the deal, the overheated lending market seized up and Kushner Companies struggled to repay its considerable loans — and to hold on to 666 Fifth Avenue. To the rescue over the next few years came the Carlyle Group, a giant private equity firm; Vornado Realty Trust, then a co-owner of two of Mr. Trump’s largest properties; and Inditex, owner of Zara, the fashion retailer founded by Amancio Ortega, the Spanish tycoon who is one of the world’s wealthiest men.

In the end, Mr. Kushner’s company survived, and he and Ms. Trump became fixtures on the international boldface-name circuit.