The businessman could face problems due to a ban on presidents appointing family members and potential conflicts of interest.
Donald Trump's son-in-law has been named as senior adviser to the President in the new US administration.
Jared Kushner, who is married to Mr Trump's daughter Ivanka, has no political experience but has been a senior figure on the billionaire's team during the campaign.
Like his father-in-law, Mr Kushner is a New York-based real estate magnate with a wide range of business dealings that could be challenged as posing potential conflicts of interest.
The 35-year-old is also a publisher of the New York Observer weekly newspaper.
Mr Trump said his son-in-law would serve as senior White House adviser.
"Jared has been a tremendous asset and trusted adviser throughout the campaign and transition and I am proud to have him in a key leadership role in my administration," he said in a statement.
His appointment could also be challenged by an anti-nepotism law that bans presidents from hiring family members.
However, a lawyer for Mr Kushner, Jamie Gorelick, argued that the 1967 law did not apply and pointed to a later congressional measure which allows the President "unfettered" and "sweeping" authority in hiring staff.
She said he would step down as CEO of the family's real estate company and from the New York Observer.
Mr Kushner would also recuse himself "from particular matters that would have a direct and predictable effect on his remaining financial interests", the lawyer added.
Mr Trump's son-in-law has continued to be influential on the transition team, reportedly playing a role in coordinating contacts with foreign leaders and shaping Middle East policy.
Mr Kushner's powerful role in the new administration emerged as Mr Trump confidently predicted that all his high-profile appointments would be approved by the US Senate.
Senators are to question the President-elect's picks for the Secretary of State, attorney general and head of homeland security, as at least nine of his appointments appear before committees from Tuesday.
The President-elect held talks with several business and media figures at Trump Towers on Monday, but the day was overshadowed by his angry reaction to an attack by actress Meryl Streep.
He also met Senate Majority Leader Mitch McConnell to discuss the confirmation hearings and repealing and replacing President Barack Obama's healthcare law.
The leader of the Senate Republicans said he expected six or seven of Trump's picks - "particularly the national security team" - to be "in place on day one".