1970s German movie, where step dad and daughter have a sexual relationship. Mom finds out and does not report it

Petra von Kant is a successful fashion designer -- arrogant, caustic, and self-satisfied. She mistreats Marlene (her secretary, maid, and co-designer). Enter Karin, a 23-year-old beauty who wants to be a model. Petra falls in love with Karin and invites her to move in.