Okay so here goes
***SPOILER***
The current chapter is cut short but this is supposed to be a rite of passage for anyone about to enter high society. ML didn’t tell FL how brutal it will be in advance and that he’s not meant to help her. She’s supposed to basically keep holding her head high as she’s harassed and mocked. It’ll be so brutal she’ll actually become cold with him. Then she meets Ezra (Lucrezia’s brother) whom she has 3 months to seduce so he can ask her to marry him. ML has feelings for her by then but dismisses them anyway because of their true status and he still won’t marry Lucrezia.
As they get familiar with each other, Ezra thinks ML has sick feelings for his “sister” so he asks our MC to marry him in front of the ML. Eventually, ML realizes that he can’t stand the idea of her being with Ezra. FL tells him that if he wants her body, then he’ll have to kneel and kiss her feet (she didn’t think he would), but when he does, she runs away crying (probably because she already has feelings for him by then. She might even have feelings for him now actually, she’s just very good at concealing them).
Fast forward to Ezra finding out she’s actually a commoner. He drops her like a hot potatoe. FL and ML finally give into their feelings and sleep together. He confesses he’s in love with her but she’s asleep so she doesn’t hear it. When she wakes up the next morning, he’s not in bed with her because he went to go buy her a ring. However she’s kicked out of the mansion before he comes back so she thinks he only wanted her body and now that he’s had it he’s done with her.
She goes as far away as she can. Meets ML’s parents. ML eventually finds her. Then ML’s parents arrange a story that says she’s a princess or a noble from some other faraway kingdom so Cesar can marry her.
Truth is she was a mermaid who met Cesar when he was a kid and gave up her tail for him. For her not to be a mermaid anymore, they both lost their memories of meeting each other
They have to be together at the end right? But I can't imagine how they'll turn to lovers from "siblings"