I did read the books but it was over 20 years ago. Do they ever explain why the grandma would cover for a daughter she hates ? Was it just to protect her husband ? And if so ....there would definatly be no reason after he died since then that would only affect Corrin's fortune at that point.
She didn't really hate her but she hated the part of her that was Malcolm. It was a complicated relationship. She hates the children because they were born of incest, but she doesn't actually cover for her daughter, she just doesn't intervene when Corrine begins sending the doughnuts. She doesn't think the children should be alive but her twisted faith won't allow her to directly commit murder. In the book, the very first time the doughnuts were brought with their daily meals she told the children sugar was bad and if she were them she wouldn't eat them. In her mind that cleared her of their murders.
When you read the prequel "Garden of Shadows" you learn that the grandmother is just as abused as the children. She is just as trapped in her own private hell as the children.