Responses
well... weirdly enough he's the ML. (just saw the spoiler from another comment just below yours, CTTO: mari-san)
He is the ml, well, he said that because he wanted a sibling since he's an only child. I mean, he's still a kid that time, his feelings would change in the future
He better not be the ml after they just confirmed they are brother and sister to each other, like before that i could kinda see some romance in the future for them but after this scene it would be weird