Doesn't have to be much sex.
I was catching up with "A well-lived life" (currently awll2 book5) and thought about the Fantasy that Steve and Stephanie had when they were younger, living together without any regrets or other outward issues.
the story should include the years as adults
Either in a scenario were they forged their ID's so that they could be married somewhere where no-one knows them, or
where one or both of them are married to another person but their main love is their sibling.