A Brother and Sister, both recovering from broken marriages decide to, shall we say "cohabit" but only as siblings, not as lovers.
Now our Hero writes Erotica, not "Pornography", and unknown to each other she has been corresponding with her brother exposing her secret love for her brother. And of course he has been doing the same to her.
Getting complicated, ain't it!
Now that tiny fickle finger of fate steps in, and they both realize how much they desire each other, and hate the thought that they have twenty some years of fucking to make up for. So they started right away; also regretting that they were too old to have any children.