Boy starts getting lucky with his girlfriend, and is thus observed. So boy starts getting doubly lucky, with his older sister. They of course both grow up and go on to have separate lives. But still they intersect. As always, enjoy! My apologies of course in the endnote.
They were thrown together as kids, and weren't sure how they felt about each other. Years later, she helps to keep him sane as he goes through the most trying experience of his life.