I think this starts with McCoy picking up Joanna after her mom and stepdad die, in a shuttle crash, I think. I actually have no memory of what happens after that, but I do know that the fic has quite a few fairly short sequels, including one where I think Joanna ends up dating Chekov, and there's something about the mirror-verse as well. I know that's not a lot to go on, but I remember loving it and I'd really like to re-read it.
I'm not sure if this is the same fic, but Kirk offers something (post on the Enterprise?) to McCoy, and tells McCoy that he'll be in his quarters at midnight, which McCoy takes to mean is his deadline to decide, though Kirk didn't mean it that way.
Thank you in advance!
ETA: Thank you
leonie-alastair who found it:
The Next Time You Say Forever by
thistlerose