I do not believe it is appropriate for a close relative of the President to undertake any assignment on behalf of a foreign government. Facts relating to the existence of any relationship should be fully and publicly disclosed.
When my Counsel informed me that according to my brother's attorneys he was considering whether to register under the Foreign Agents Registration Act and report his activities, I urged him to register and make a full disclosure. I note from the registration that he is not presently engaged in any activities on behalf of Libya and has no activities on its behalf under consideration.
Jimmy Carter, Billy Carter's Activities With the Libyan Government Statement by the President. Online by Gerhard Peters and John T. Woolley, The American Presidency Project https://www.presidency.ucsb.edu/node/251023