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). However, this feelings would soon change cause of Anisa herself. And as we all know, he'll love her. Also, there is a spoiler that he already knows she is not his sibling from the start cause he felt it when his mother and younger brother (yes, it was supposed to be a boy) died.

i wanna be spoiled, gimme some spoilers pls