Let me try and make this sound not totally like a cheesy highschool AU Karkat would watch as a RomCom. Feferi and Eridan have been 'highschool sweethearts' for a few years, Feferi doesn't feel she's the right one for him, Meenah, her older sister, helps her arrange an agreement with Sollux to 'take Eridan off her hands' for her. ( answers to comments. On AO3. I have a beta reader.)