As I understand it, Fah was in love with Sea's mother, but as it was unrequited (as in "I'm not into women, sorry") she transferred her affection on Sea, with the blessing of her mother, as in "yeah, go ahead, do what you want with my daughter, I trust you more than random boys. Just stop smoking, okay?"
So Fah, after hesitating, stopped smoking and having sex with Sai and said she will wait for Sea to graduate to screw her.
It's wrong on many levels. But as it is amateur work, I won't complain (too much) about the lack of coherence.