Your character is home now, out of the service do to some reason or another, I'll leave that up to you, and he calls his best friend, asking if he can crash for a bit until he finds a place of his own. But he is expecting a baby and has two other little ones at home so he has no room for YC. So he calls his sister who just so happens has an empty room in her apartment as of a week ago.
When YC knocks on the door, he expects the annoying little sister he remembered growing up with, even though those last couple of years in high school, she had done some growing up. And he had noticed, but nothing could happen there. This was his best friends little sister. Luckily though, she had kept up with her tomboyish look. Hell, just to keep her away from him, he had taken to calling her Max and that had pissed her off to no end.
But what opened the door on that rainy evening when he had arrived on her doorstep, was not at all what he had expected. She had grown out of that tomboy faze and had grown into one hell of a gorgeous woman. And he was expected to live under the same roof as her?!
That is the gist of my plot. If you are interested, PM me please and we can chat a little more on the idea and where it will go from here.