Have you ever noticed in these Arthur books where the Tibble Twins appear:
Arthur Babysits
D.W.'s Library Card
Arthur: It's Only Rock 'n Roll
Arthur and the 1,001 Dads (Arthur Chapter Book)
Arthur's Birthday Surprise
D.W.'s Guide to Preschool
D.W.'s Guide to Perfect Manners
Arthur Turns Green
...the Tibble Twins are human looking rather than animal/bear looking?

Yeah it's quite surprising. Their grandmother also appears to be human looking as well.