I don't think Tsuyu here's out of character at all. She can't speak her mind in normal situations because she's shy, but here she knows she wants to have as much fun here with Onii-san. With Chie gone in a longer period of time than nornal (being that normally she's only temporarily outside of the room or whatever), Tsuyu is opening up little by little. It's just her character growing, yknow. She is like fourteen(ish?) after all.