In the top right corner, Illya is squeeing over Shirouko's cuteness in her own body, saying "Kyaa! Big sis- big brother is so cute!"

I'm not sure about the other three, especially Rider in the top left corner. I suspect based on her personality in Hollow Ataraxia she probably is saying something snarky about Shirou's pathetic appearance, especially since she's placing emphasis on her comment. Meanwhile, Rin's trying to hold back her laughter in the bottom right, and Sakura is apparently lamenting something about her former sempai.