The crossword solution for the term 'ABBY's Sister' is ANN, which is a 3 letter word. ANN is ABBY's sister, and they are both characters in the popular children's book series, 'The Magic Tree House'. The series follows the adventures of Jack and Annie, two siblings who travel to different places and times with the help of a magical tree house. ANN is Jack's twin sister, and she often joins him on his adventures.