From @abumperprize:
“I also felt like Cora was doing some Parenting of Rose that we never saw. Rose transformed from a selfish flapper, sleeping with a married man into a poised young woman who helped care for Russian refugees. I don't know what parenting a girl who's nearly an adult looks like, but I guess neither does JFellz, but I also think Cora was doing it and doing it well.”
It wasn’t just Cora who did some parenting. So did Mary, Edith and even Anna. She was a selfish brat, for sure, but she also seemed to want to be helpful and kind. It’s just that Susan had created an environment in which Rose was always corrected, even if she wasn’t doing anything wrong. If you’re damned if you do as well as don’t, she chose to do whatever she could to hurt her mother and herself the most. Once she wasn’t under the constant barrage of criticism, things changed for her.
She was also allowed to make mistakes. Susan seemed almost pathelogically afraid of what rose would “do” and sought to control her so she wouldn’t have a chance to face consequences. It means Rose was only confronted with her mom’s consequences, not the rest. Both by going to the dance and pretending to be “regular” and thoughtlessly bringing down Matthew’s gramophone, she was given a front row seat to the results of her actions. She was beginning to realize she didn’t live in a vacuum and that the things she did impacted people other than herself and her mom.
In short, she came into a family who treated her like a person rather than a show-pony and she was able to come into her own. Her natural sweetness was allowed to breathe and wasn’t stomped out by Susan.