Responses
I don’t think he sees her as a sister. He is only playing it off that way because Rosie calls him brother. He knows perfectly well she isn't adopted, only was given the family name, so which means his obsession lies further beyond the capacity of sister complex.
Her brother creeps me out. Although his sister complex is not as bad as in the other manga but it still is creepy. And for someone who was like the greatest mage or whatever, she’s so easily fooled.