On the off-chance that Hillary becomes the next president... There is precedent...

My mom was a History major in college, so naturally, my brother and I were exposed to LOTS of history. More than one daughter, and indeed, other relatives of presidents, served as First Lady throughout our nation's history. The linked website below also includes individual events (such as Chelsea Clinton accompanying her father to Australia while Hillary campaigned for her senate seat), and I disagree with their inclusion, but it IS a comprehensive list.

http://www.firstladies.org/blog/first-ladies-never-married-to-presidents-the-other-women-of-the-white-house/

I recently read about it, plus possible bi or gay president(s). I doubt Lincoln was bi, but it's not like history doesn't repeat itself in various permutations. What's remarkable is that we can make educated guesses that (it) will happen again & it does, despite historians jumping up & down making warnings.