Are Mother Winter and Mother Summer, respectively.
Except Simon met with Baba Yaga several times, and she offered him power and knowledge and he refused, and did not choose to be her servitor. He also was certain she was NOT fae. We have a WOJ saying that Mother Winter is, for all intents and purposes, Baba Yaga. So we have a contradiction.
My belief is that Baba Yaga and Mother Winter, and Baba Vasilisa and Mother Summer, are mantles. What is underneath and wearing them is harder to say.