
Dietrich doesn't really see Anisa as his younger sister. He keeps saving her cause he has Baal, a demon king. He needs someone "precious" to him to give as sacrifice when he becomes the Grand Duke later on. So Baal is going to eat Anisa (whole) and it would give Dietrich more power, kinda like a medium. This is because they believe that you have to throw away any thing that makes you human (and get closer to the devil) and love and family is what makes human, human. That's why the title is "the PRECIOUS SISTER of ...", cause Dietrich literally has to show he has someone he cares for for shows only (for his father).

dun worry HAHAHA that was just his first intention but while his heart is now slowly getting soft for her. I read the following chapter and that's actually where he says about the sacrifice. but he'll start to feel jealous when others are talking to anisa. he's like slowly caring for her for real but he still doesn't want to admit it yet
spoiler if the manga follows the novel's actual plot
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the demon that detriech called will become a play doll / friend for the baby and the demon is so cute bcs it treat her fine