They thought they could keep it secret; that neither would make any demands on the other and it would be perfectly safe. Most "family" affairs are interwoven with family dysfunctions and buried resentments.
Neither Bill nor Tina, his sister-in-law, looked seriously at the issues in their respective marriages or inter-locked families; or even how dangerous it was.
They enjoyed talking and looking forward to time together.
They spoke on the phone frequently and lingered around afterward working on the project.
She had long felt unaffirmed, ignored, and disregarded by him.
His adamant refusal to go to couples therapy pushed her into acting upon her anger.
Aside from the challenge of remaining on the chaste side of the sexual borderline, such "lovers" must hope that their primary partners continue to believe they're telling the truth.