Responses
Don't forget she's tiny according to their standards. He probably thinks she's like 2.
I was looking at the raws (I don't understand what they're saying) and it really felt like he's thinking "I won't let myself be charmed by her".
Ruenti is definitely thinking something along the lines of "How long was I gone from home to have a little sister this big...?!!!" (￣∇￣")