What is the nickname of Arthur's little sister?

Arthur's little sister is nicknamed D.W.

D.W. is short for Dora Winifred Read, Arthur's younger sister in the Arthur books and television series. She is a preschool-aged child when the series begins, while Arthur is an older elementary-school student.

D.W. is one of the show's most prominent characters, often appearing in stories about sibling rivalry, jealousy, imagination, and family life. She frequently annoys Arthur, but their relationship also shows affection and loyalty beneath the bickering. Her full name is occasionally used, though most characters—including Arthur—usually call her D.W.

The initials are easy to confuse with invented alternatives such as R.W., P.W., or B.W. D.W. is not Arthur's youngest sibling: Kate Read is the baby of the family. D.W.'s character also received expanded attention in episodes and stories centered on her preschool experiences and independent personality.