First, the characters were introduced as part of the Random House Beginner Book series. These books were designed for children just learning to read and so featured very simple language emphasizing words that were phonetically easy to sound-out and which could be visually matched with and un-confusingly associated with the elements of the illustration. So the initial three family characters were, “Papa,” “Mama,” and “Small Bear.” More complex words like, “Mother,” “Father,” or a specific name would not have have fit this function. Later, when the characters appeared in storybooks with more complex language, this naming model was continued for consistency. When Sister Bear was introduced, it would have been incongruous for Small Bear to suddenly have a specific name so he became “Brother” and the new baby, “Sister.”   Second, the Bear family was introduced in 1962. This was the tail-end of a social and cultural period where it was quite common for family members to refer to each other by their family positions rather than their names. People really did refer to each other as “Brother,” “Bro,” “Sister” or “Sis.” So the Berenstain Bears books reflected this origin. But this naming convention of the early 20th Century nuclear family was not applicable to new characters as they were introduced over the years.