In D.W.'s 4th birthday flashback in "D.W.'s Perfect Wish", her mom is shown to be pregnant with Baby Kate. However, in the earlier episodes "Arthur's Baby" and its follow-up "D.W.'s Baby" from Season 1 that are backstory episodes about Baby Kate being born into the family, she was three years old during those events, and Baby Kate should be one year old in the present day setting.