The New York Times Editorial Board.

President-elect Donald Trump’s decision to appoint his son-in-law, Jared Kushner, as a senior White House adviser very likely violates a federal anti-nepotism law, and shows again how little he seems to care about the legal and ethical obligations of the office he is about to assume.

The language of the law is clear: No federal official, including the president, may hire or appoint a relative, including a son-in-law, to “a civilian position in the agency in which he is serving or over which he exercises jurisdiction or control.”

There’s a good reason for anti-nepotism laws, versions of which are also on the books in most states. Government officials seek informal advice and counsel from relatives all the time, but when they appoint or hire those people, they undermine the public’s faith that important posts are being filled with the best possible candidates. And when relatives get security clearance to view classified information and sit in on high-level meetings, it upends delicate dynamics, as senior staff members keep their mouths shut rather than contradict a trusted relative of their boss. Even if Mr. Kushner is technically subordinate to others on the White House staff, he is always first and foremost Mr. Trump’s son-in-law.

The scope of Mr. Kushner’s responsibilities is not clear, but it could be extremely broad. He was by Mr. Trump’s side throughout much of the campaign, an influential voice with impressive contacts. At one point he arranged a meeting between Mr. Trump and the Israeli prime minister, Benjamin Netanyahu.

But Congress, which passed the measure in 1967 partly in response to President John F. Kennedy’s appointment of his brother Robert to be attorney general, was aiming to curb the negative effects of nepotism throughout government. Concerns about nepotism are, if anything, stronger in a White House appointment, where multiple close advisers and fragile hierarchies can easily become snarled by family allegiances.

In addition to being related to Mr. Trump, Mr. Kushner, a 36-year-old real estate investor who is married to Mr. Trump’s eldest daughter, Ivanka, lugs behind him other significant liabilities. Among these are a complete lack of experience in politics or government, and a boatload of conflicts arising from his family’s vast real-estate holdings.

As recently as November, Mr. Kushner met with major Chinese investors over the redevelopment of his family’s flagship property, a Midtown Manhattan skyscraper he purchased in 2007 for $1.8 billion. Mr. Kushner has said he will sell off his interest in that building and other top investments and resign as the chief executive of the family business, Kushner Companies — but like Mr. Trump, he is keeping those assets within his family, creating what one lawyer called a “shell game.”

Mr. Trump has already mocked concerns over his own conflicts of interest, saying that if the president does it, it can’t be a conflict. After riding into office on promises to “drain the swamp,” he now appears equally untroubled by the real dangers posed by nepotism, and uninterested in following a sensible law.