@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.