User blog comment:IamJakuhoRaikoben/Chapter 492: Elder Sister and Younger Sister, Review./@comment-4449950-20160705100735/@comment-9595671-20160705175211

I think the best way to explain how Mira managed to get Algeria's "soul" is explained by Atlas Flame and Zirconis.

Even after Algeria was cut, it's soul still lingered for a bit. Though it could be possible due to her magic she absorbed some of Algeria's power while everyone was still trapped.