Yoon Ma Ri has literally and un-literally gone over to the dark side. In Episode 12, Ma Ri finds out Min Sun Jae (played by Kim Kang Woo) has been injecting her with hormone shots to simulate pregnancy behavior. Up until this moment, she has suspected some of Sun Jae’s evil behavior, but she didn’t think he would harm her. This is why she hasn’t turned him in to the police and hasn’t done anything to harm him. She’s been trying to keep the family together. However, she breaks down after she finds out she isn’t actually pregnant. She intentionally hurts Sun Jae by kissing Cha Ji Won (played by Lee Jin Wook).
See her transition here: