Ruth Fremson/The New York Times
Jared Kushner, President-elect Donald J. Trump’s son-in-law, at Trump Tower in Manhattan on Sunday.

WASHINGTON — Jared Kushner, the son-in-law of President-elect Donald J. Trump, has spoken to a lawyer about the possibility of joining the new administration, a move that could violate federal anti-nepotism law and risk legal challenges and political backlash.

Mr. Kushner, 35, the husband of Mr. Trump’s eldest daughter, Ivanka, and an influential adviser to his father-in-law during the presidential campaign, had been planning to return to his private businesses after Election Day. But on the morning after Mr. Trump won, Mr. Kushner began discussing taking a role in the White House, according to two people briefed on the conversations who requested anonymity to describe Mr. Kushner’s thinking.

Mr. Trump is urging his son-in-law to join him in the White House, according to one of the people briefed. The president-elect’s sentiment is shared by Stephen K. Bannon, the chief strategist for the White House, and Reince Priebus, who was named chief of staff. Mr. Kushner accompanied Mr. Trump to the White House on Thursday, when the president-elect held his first in-person meeting with President Obama.

Mr. Kushner may be as influential, if not as well known, in Manhattan as his father-in-law. Mr. Kushner is a real estate mogul in his own right who, like Mr. Trump, runs a company founded by his father. His influence extends to the news media: He owns The New York Observer. And his family’s wealth rivals that of the Trump clan, which he joined in 2009 when he married Ivanka.