Robert F. Kennedy Jr., an independent candidate for president, is being questioned by the Federal Election Commission for payments his campaign made to his daughter-in-law last year, according to a new letter first reviewed by CNBC. The FEC sent Kennedy's campaign a letter on Tuesday, asking for details about "payments to members of the candidate's family" between July 1 and Sept. 30. The FEC threatened to take legal action against the Kennedy campaign if the payments to family members did not reflect the fair market value of legitimate campaign services. Although the letter doesn't name the family member, Kennedy's daughter-in-law, Amaryllis Fox Kennedy, is the only family member who received payments from Robert F. Kenned