It's already revealed in the spoiler-cut.

While Fern and Rosemary were raised as twins, they were separated at age five. So while there are some scenes of them interacting (which were by far my favorite parts of the book) it's not really about their relationship, because that ended when Rosemary was so young.

Rosemary was too young for most of the relationship to really get exactly how different Fern was at first, and they parted just as she was beginning to understand that. Rosemary was also influenced by Fern, enough that her body language was weird to her school classmates. She thought Fern could do a lot of stuff better than her, so she was jealous, and hated her for making Rosemary an outcast. They had a love-hate relationship going.

Fern killed a kitten that Rosemary gave her, and Rosemary was upset but also saw an opportunity to get rid of Fern. So she told her parents Fern scared her, which Rosemary thought was a lie but which probably was also true to some degree, and Fern was sent to the lab. Rosemary then blamed herself for the whole thing.