"But as commentators were quick to point out, it wouldn’t be the first time for someone other than a presidential wife to act as “first lady,” a role with no formal set of rules or established parameters. In the early 19th century, two widower presidents turned to family members to act as official White House hostess: Thomas Jefferson enlisted the aid of his daughter, Martha, and Martin Van Buren turned to his daughter-in-law, Angelica. Between 1857 and 1861, Harriet Lane played the part—and played it exceedingly well—for her bachelor uncle, James Buchanan.
To those who would argue that Donald Trump is neither a widower nor a bachelor—he has a living and breathing wife—here, too, there is historical precedent. For several years during her husband’s tenure in office, Eleanor Roosevelt declined to fulfill the traditional role of White House hostess, preferring to devote her time to public and political work. Her daughter, Anna, stepped into the function.
Much about the Trump presidency seems likely to break with two centuries and more of historic precedent, but history by no means suggests that Ivanka Trump can’t or shouldn’t act as first lady, particularly if her stepmother prefers to maintain a private life, out of the public spotlight (as should be her right). But with the role come great scrutiny, a very high ethical bar and stringent obligations to the public. Given her family’s frequent and indecorous habit of mixing business with politics, it’s not at all clear that she’s up for the task."