Responses
Oh dam
Does he sacrifice her in the end though? Does he grow to like her a lot since you said he started to doubt himself?
He doesn't sacrifice her. And indeed he gets to really love her because she is all he has.
Yay, that's a relief!! (๑•ㅂ•)و✧
Spoiler for those who are worried about inscest
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He is only taking care of her because he is planning to sacrifice her in order to win the game and become the duke. The final obstacle is to give the most precious thing/person to the demon in order to win, so he wants to fool him. Therefore he is protecting her to use her for that (the major sacrifice). That would explain why he saved her and replaced the baby his mom lost back then. Also what I am saying is the original plan he had... as time passes and she grows older he starts to dubt himself. But he has never see her as a sister but a mere sacrifice... never! And she doesn't either. He is just a way she can survive. So yeah no siblings relationship here.