
It doesn't explain it well here but in the novel, it's explained that they never had a particularly good or close relationship when younger because their dad would favor Juliette. At some point when younger, Sigurd (is that how you spell it lol) left the family to train as a soldier. He was taken under Jolga's wing and he even wanted to be Jolga's son (unknowing of his parentage at the time). He also hated his father and sister and basically considered them strangers because of everything he heard about them. They were evil in his eyes, and he wasn't happy about being Juliette's guard at the beginning because he thought he would have to go through hell because of them. However, due to Juliette being instructed by their father to just be nice (as she had always been encouraged to be mean and spoiled to get ahead) Sigurd began to have a better relationship with Juliette. Is it still weird? To some yes to others no, but in the manga it is harder to tell that they weren't really raised thinking of each other as close/family, especially when Sigurd left for the military to escape the family.
I mean like... He still considers the sis as his sister, no??? And do they know they aren't blood related anyway? Like genuine question, do they or do they not?also Ngl I kinda saw this happening when the bro was shown to care alot for sis from that one chapter. They grew up as siblings so this shit is so fucking weird. "Oh but they aren't blood related anyway!" It's still incest if you think about it logically