Confused about the twins

Yayoi and Kasumi. I know Kasumi became a breed and ripped people's faces off because she blamed herself for what happened to Yayoi and all that. So my question is, at the end when Yayoi has a new unburned face, had she and her sister switched bodies somehow, or did doctors surgically attach her sister's face to her head?

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"House. My room. Can't walk. My medal. My father. Father, don't!"