The twins look back at her with looks that vary from total innocence to OH SHIT NO DON'T TELL HIM WE WILL NOT TELL THEM and back to total innocence.

"And we weren't wrong." Martin adds "You proved to be most interesting."
"To us most unexperienced alumni of life."

They barely disguise their surprise (at least in the eyes of their sister) at the gesture, which prompts them to exclaim in unison:
"Ah, so those flowers were from you, Monsieur!"