To date, money from Mr. Kushner’s immediate family members represents less than $5 million of his venture funds. Jared is now divesting his shares in Thrive because of his move into the White House. The Trump family has not invested.

That is not to say that Joshua Kushner is not close with his family. He is tight with his parents and two sisters, as well as with Jared (who is four years his senior) and with Ms. Trump, whom he refers to as his sister, not his in-law.

While the brothers are known by friends and colleagues as ambitious workaholics with ties to power brokers on both coasts, in many ways their paths have diverged. Jared Kushner has emerged as an increasingly public figure since becoming the face of the family’s real estate firm, Kushner Companies, and buying the publisher Observer Media. That company’s newspaper, The New York Observer, has mythologized the city’s media, real estate and political power brokers for a generation of New Yorkers. Jared is also often photographed at society events with Ms. Trump, and is now frequently seen in the background of meetings with the president-elect.

But Joshua Kushner has maintained a lower profile. While he has dated the model Karlie Kloss for four years, he is rarely seen out with her. He does not party or drink. Unlike most venture capitalists, Mr. Kushner also does not blog, and he posts to Twitter infrequently.

“Most moguls love to name-drop and tell you about their vacations and appeal to you with how celestial their orbit is, but that’s not Josh,” said Patrick Collison, the chief executive of Stripe, a mobile payments start-up in San Francisco that Mr. Kushner has invested in.

Mr. Kushner declined to speak on the record for this article, and a spokesman for Mr. Trump declined to comment. In an interview, Ms. Trump said that “Jared and Josh have a very special relationship defined by tremendous love, admiration and mutual respect.” She declined to talk about politics.

An Investor’s Roots

Joshua Kushner grew up in New Jersey, where his grandparents, Holocaust survivors who immigrated from Europe in 1949, had built apartments. Their properties turned into a $1 billion apartment development business under his father, Charles, who was sentenced to two years in prison in 2005 for making illegal campaign donations, witness tampering and tax evasion. Jared later expanded the family business and bought luxury office properties in Manhattan.