They have not made any appearances in the book.
Miss Peregrine's Home for Peculiar Children
In Miss Peregrine's Home for Peculiar Children, the girls have been mentioned once and a picture is shown of them in the first book. Jacob wonders why the girls are standing with their backs turned towards the camera because portraits cost money back then. Apparently the twins are telepathic, but some people assume the girls both have a backmouth, or even a backface, which is why they are shown with their backs to the camera.