Beatrice's soul is in current Raeliana's body. I'm thinking it's kind of like WMMP, where Athy got reincarnated twice or something like that.
This theory makes sense
Was Athy reincarnated twiice in WMMP? I can't remember. Anyway, Iosa just said she needs to kill the current Beatrice in order for her to live. I guess her current soul is unstable in her current body because the original owner of her body is in the same world. That may also be the reason why the current beatrice keeps on trying to kill her because she needs her old body to perish in order to for her soul to completely stay in her current body. Its also possible that original beatrice's soul maybe inhabiting the MC's original body in her world. Like the original beatrice maybe isekai-ed in MC's world.
This is my theory: the old realiana asked the dead mage (or sister in catholicism) to help her get into beatrice's body coz she have a better life than her and to avoid conflict (coz she knows that the original beatrice is smart and tenacious) they took another soul from another world to put it in her body, thinking that since she's suddenly thrown in an unfamiliar world she would just die quietly. She didn't take into account that the new soul would have full knowledge of what would happen and is even more tenacious and smarter than beatrice. But the real question is... what happened to beatrice's soul?