What happens when you're not the one that gets Isekai'd?

Noem knew in the bottom of his heart that his sister Mona wasn't going to make it. Years on life support that he funded through... not quite legitimate means had deteriorated her body and her mind in equal parts. But one day, after a skyrail heist gone horribly right, he finds something wrong in her. More specifically, someone else. Someone from somewhere called 'Earth'.

Someone who's supposed to save the world. And someone Noem's supposed to just forgive for killing his sister and walking around in her skin.