
It's because of the tradition, she has been told she would marry Abel for the past 10 years.
It's a pity that Abel decided to leave the village without clearing this situation first.
Gizel has been by his side for her whole life afterall, he should handled the situation better.
It would be sad if she became twisted because of this. She is such a cute little sister I honestly adore her.

I didn't mean it like that, it's just that very often in manga especialy in isekai the " little sister has crush on mc nii-sama " trop is there for fanservice and, well I personally think that romanticize this is gross, and incest isn't taken seriously in those case.
But I like this manga because they didn't use it like an harem trop, they used it as an important part of the scenario. I get that it is there tradition, they don't have problems with genetics because of magic power, it doesn't include incest parent/child and there is a limit for age gap.
And I really like that the mc can't agree to this. Because of his memories from his past life he can't have the same worldview as them, witch make sense. A lot of isekai heroes are quick to throw away there values from their past and be like :
eh ! I guess I don't mind slavery anymore since it's normal in this world. I'll just go buy some slaves. ╮( ̄▽ ̄)╭

I made this comment not because I support incest relationship but rather because I was disappointed he left her. I'll make this clear.
1. It's not like we can push our morals to others. We are against incest because that's what our society taught us, but our times and theirs are different. What's a norm for them is queer for us and vise-versa. Logically speaking isn't it the same as homosexuality? Polygamy-or should I say harem? and others?
2. It's a story. Doesn't mean because we are reading and enjoying it, we apply it to reality.
3. And I want her back soon, I can see potential (▰˘◡˘▰) like I said mah Gal is gonna be a demon king.
But Yandere little sisters are cute~ it was such a rare specimen. I'm kinda disappointed he left her. And here I was misled by the cover~ I thought they would be on a journey together :< mah gal's gonna be a demon Lord, just you wait.