Rebecca's warnings didn't help, because Ethan postponed the wedding and began seeing both woman, getting disinherited as a result. After Gwen returned her engagement ring, Rebecca was determined to get not only her daughter, but also herself in the Crane house, and get revenge on what she believed to be gold-digger for scheming to break up Gwen and Ethan's relationship in the first place. It's possible Rebecca saw her husband in Theresa, as he had once married her for her considerable fortune.