This looks like Reimu is saying "Don't peak!" and Rinnosuke is thinking something along the lines of "Why would I?". But i guess that is just a little too obvious from their facial expressions.
Rinnosuke's reaction is becoming complexly emotional. This is...
ReiRin Maximum Drive!!
Rei: "Not that I'm ungrateful for you sewing up my clothes, but you'd better not peek while I'm changing."
Rin: "Mm? Oh, yeah..."
Rei: "Are you even listening?"
Rin: "Yeah..."
Rei: "...it wouldn't kill you to show a little interest..."
