My theory was right all along. The witch is Elsa. I did thought tho that could be Olive from future cuz they had similar hair colors but Elsa was first theory and they confirmed in this chapter with this foreshadow that something will happen to Evie.
I would have thought they saw each other naked at least once, since they sleept, change clothes and more in the same room and that. Plus, it's Elsa we're talking about, hard to think she would let the chance pass...