Thanks for your feedback, Tangent101 and Zhangar. And as this is the DM thread, you do not really need to use spoiler buttons here.
It gives me enough ideas to develop this plot line even further. Whether or not for immortality or youth, Baba Yaga needs her daughters for some purpose connected with her own life.
I have been thinking about the origin of the daughters. My ideas thus far are that they are the children of Baba Yaga and Russian tsars (and other, even older rulers), linking her and her family firmly to Russia. And they might indeed be in stasis or in a different timeline before they become the rulers of Irrisen (The idea with the tsars is that the rulers are one with the land, as was often seen in pagan times, such as with king Arthur.)
I am also thinking of linking Russia's health to Baba Yaga's, which gives her an extra reason to rejuvenate. I am linking her to the "mother Russia" idea. This fits nicely with the idea that "Irrisen is a crucible". Irrisen is only there to keep Russia alive.