Mr. Kushner, who declined to be interviewed for this article, has hired a leading Washington law firm, WilmerHale, to advise him on how to comply with federal ethics laws should he join the White House staff as an adviser to the president. The firm has concluded that one potential sticking point, a federal anti-nepotism law, is not applicable, though not all ethics experts agree. While the law prohibits federal officials from hiring relatives for agencies they lead, Mr. Kushners lawyers argue, among other things, that the White House is not an agency and is therefore exempt.