@katashu and @Wolvenworks I don't think it works on the dead yet, maybe later, that girl was still alive, she was frozen just on the brink of death. My guess is that he uses it on all the things he kills to make sure the corpse is always pristine though, like that other party's piece of shit kill.
Edit: just occurred to me, in that closet was Shroedinger's loli, she was both alive and dead.
@Acidroach I wouldn´t call him a cunt for that. Look, from the point of the sister, she just got slashed by a monster one second and wakes up in a strange room in the very next moment. Her first words even indicate that she believed herself to have died. He had no way of knowing how she would react, so not telling was more out of consideration for her than being a dick.
After all, he spend 25 years (since people in medieval-like worlds die sooner than nowadays, you could say 25 equals pretty much his entire life) to bring her back without knowing if it had any chance of working. That should be proof enough that he cares deeply for her.