Irisviel, kudos to you for being a true mother to your child, and encouraging Illya to go and seek what's truly important to her.
But, I do understand Illya due to being just as impressionable as her, and it was rough for her learning those words from Miyu herself. Nonetheless, Miyu said them to protect Illya, because deeply she truly regards Illya as a friend as well; she, at last, openly said that she treasures Illya as her best friend and her first one.
BTW, Saber Miyu rocked against Black Berseker, but the one truly crushing the Heaven's Feel was Archer Illya.
