My best friend's brother

Thanya and Gulf have been best friends since kindergarten and Gulf often spends most of his time in Thanya's house since his parents are always travelling and he hates being in the house alone with just servants, It's just way too lonely.
Thanya has a brother named Mew and they don't get along since childhood, they really hate each other.
What happens when Gulf and Mew (Thanya's brother) have sex one night and both can't seem to forget it although they promised each other they would.