When I got home I went straight on IMDB to see who played Amy's sister in the movie. I didn't recognize her from any films that I had seen but thought that she was excellent in the role. Stunningly beautiful but quietly unassuming, she was perfect. Her name is Brie Larson and although she has been in loads of films I haven't seen any of them. I'll be watching out for her in future films!
Here's the obligatory cinema selfie of my twin and I!