He has also lost some of his closest allies: two aides, Reed Cordish and Josh Raffel, recently announced their departures, as did Gary D. Cohn, the president’s economic adviser. Still hoping to make an impact on global affairs, Mr. Kushner has turned his attention from the Middle East stalemate to American relations with Mexico.

Charles Kushner is adamant that his family remains united in the face of their difficulties. But friends say Jared Kushner’s sister is distressed by investigators’ focus on her, and there are tensions between Jared and Josh over Mr. Trump. That Josh Kushner, 32, has made no secret of the fact that he did not vote for Mr. Trump upset his brother, several friends said. Voting records show Josh Kushner did not vote in the 2016 presidential election. Josh’s spokesman, Jesse Derris, said the brothers “are just as close as ever.”

Tales of his distaste for the Trump administration continue to surface, most recently at a January party at Oscar, a health insurance firm that he helped found. According to one attendee, Josh Kushner listed the year’s challenges, concluding with a laugh: “We survived Donald Trump. Don’t tweet that. Really, don’t tweet that. I’ll get in so much trouble.”