It's a movie night with HIS sister, that he will attend. Setting aside the question of why she's responsible for securing snacks for a night out with his sister, how much do you want to bet that, if she had bought budget snacks (instead of baking a cake and buns), he would have yelled at her for embarrassing him in front of his sister?

She can't win. She plans for, shops for, and cooks homemade food for a night with his sister. Instead of "thanks for doing all of this and WOW you baked a cake!" She's "in trouble." It's obnoxious. She should separate their money and put him on shopping duty from now on. Or better yet, break up and buy whatever she wants with her money.