This Sickly Beauty Has Incredible Strength

There Is a Book About a Heroine and a Mistress. the Heroine's Mother Is as Powerful as the Ceiling but Stupid.

There is a book about a heroine and a mistress. The heroine's mother is powerful but stupid, but the mistress is the comic relief in the book. I seem to have forgotten her surname, Song. The heroine and mistress both have two characters, and the heroine takes her mother's surname. Can't find it