What is the name of Arthur's baby sister?

Arthur’s baby sister is named Kate.

Kate Read is the youngest child in the Read family and the younger sister of both Arthur and D.W. She is usually portrayed as an infant or very young child, making her role different from D.W.’s preschool-age storylines. Kate appears in family scenes and often adds a baby’s perspective to the Read household.

One of Kate’s distinctive traits is her ability to communicate with animals. She can understand and interact with characters including Arthur’s dog Pal, Binky’s baby sister Mei-Lin Barnes, and several other animals. As Kate grows, she gradually loses the ability to communicate with some of them while becoming better able to understand human speech.

Kate is sometimes called “Baby Kate,” especially in descriptions of the series, but Kate is her canonical given name. D.W. is not the baby sister: Dora Winifred Read is Arthur’s four-year-old middle sister and Kate’s older sister. Pal is Arthur’s dog, while Muffy is one of Arthur’s classmates and friends.