From a December Washington Post article on the potential of Ivanka Trump being the first lady:
First ladies aren’t always presidential spouses. In fact, two early uses of the title refer to the beautiful, popular Harriet Lane, niece of James Buchanan, the only lifelong bachelor president. She was an able hostess who, not long before the Civil War, arranged for Northern and Southern guests to be seated apart at a White House function in order to keep the peace. Harper’s Weekly called her “Our Lady of the White House,” and Frank Leslie’s Illustrated Newspaper captioned Lane’s picture thusly: “The subject of our illustration … may be justly termed the first lady in the land.”
So as we learn that Ivanka Trump, Donald Trump’s older daughter, arranged a meeting between the president-elect and former vice president Al Gore; that she and husband Jared Kushner are reportedly house-hunting in Washington; and that Ivanka is rumored to be looking at White House office space, it’s pretty fair to say she isn’t breaking completely new ground. There’s no job description, so like other first ladies, Ivanka can define her position — and it looks like the gig is hers — in a unique way: advocating for nonpartisan causes, as Laura Bush did with children’s literacy and Michelle Obama did with nutrition, or setting up in the White House’s West Wing as a de facto policy adviser, like Hillary Clinton.
I never knew the first lady didn't have to be the president's wife.