Why did Ruby's mom keep Ruby away from her older sister?

I don't think Ruby's mom had any right to do this. Cora was trying to get in touch with her sister after she left for college and Ruby wanted to talk to her. But, whenever Cora tried to talk to Ruby their mom would say that Ruby didn't want to speak to Cora. I think she did this because she didn't want to be alone. Ruby's mom knew that if Cora and her sister talked that she would take Ruby away. So to prevent this Ruby's mom never let the two sisters speak. When Ruby found out about this she was really upset. She had thought that Cora was the bad guy because she never tried to contact Ruby. But, really it was Ruby's mom that was the bad guy and that was hard for her.