His smirk is cause he's realized how she feels for him/around him in that moment
He knows theyre not the original siblings and that he's actually not related to her like she might still think
He smirks because he has interest in her and her reaction shows she has interest in him thats non-familial suggested by her blush, the look on her face, and the tone in which she speaks
His face in a way looks almost arrogant
He knows theyre not the original siblings and that he's actually not related to her like she might still think
He smirks because he has interest in her and her reaction shows she has interest in him thats non-familial suggested by her blush, the look on her face, and the tone in which she speaks
His face in a way looks almost arrogant