After My Sister Died, I Returned to a Wealthy Family on Her Behalf and Abused the Scum Like Crazy

I feel that the story concept is okay, but many places are not explained clearly. More of the portrayal is about playing games, but the feelings of the mistress and the belly-tightening child are not shown, which is a pity. There is also the fact that she and her sister are twins. Why the heroine gave them away while her sister did not is not explained clearly.