I would love to find an older brother for an incest line with as much forbidden-love as there is smut. Perhaps something where they were involved when they were younger and because he loved her he thought the right thing to do was to leave and let her have a healthy life. They haven't spoken since, but now 5+ years later he's come back and shows up at her door to apologize, and they both realize the connection is still there. (Comments are screened, if interested feel free to make a custom or ask for one.)