I read this years ago, cannot remember the title or author.
MC is happily married, wants to start a family. She is very driven and focused on her career.
Wife's twin sister comes to stay. She is the opposite of MC's wife- Lazy, selfish, etc. and blames sister for her failings.
SIL tries to seduce MC by pretending to be MC's wife home early from a business trip. She puts on wife's perfume and slips into his dark bedroom.
The MC catches on when he feels her full bush. She did not know her MC's wife keeps herself waxed bare.
The wife gets home and finds out, decides that her husband will knock up her sister and she will be their servant/wife.
Does this ring any bells for anyone?