User blog comment:B214/Question and Answers 6/@comment-73.57.25.92-20171215221621/@comment-29963364-20171215223245

We already know that their not blood-related... at all.

Griselda is Xenovia's boss (not anymore, since she became a Devil), mentor, and legal guardian. Their only considered sisters because that's how Griselda introduced herself when she attended Xenovia's "Parent-Teacher Conference."