The term 'Sister of Peter Rabbit' refers to two characters from the classic children's book, 'The Tale of Peter Rabbit' by Beatrix Potter. The two sisters are Flopsy (6 letters) and Mopsy (5 letters). Flopsy is described as a gentle and timid rabbit who loves to eat lettuce and carrots. Mopsy is described as an adventurous and mischievous rabbit who loves to explore and play tricks on her siblings.