Mariah and Miranda Shillaber are students at Stoneybrook Middle School in the same grade as the older BSC members. They are twins.
The twins' primary roles in the series are as Mary Anne and Kristy's tablemates in the cafeteria before the BSC begins eating lunch together at the beginning of eighth grade. Kristy had made friends with the twins before Mary Anne, and they had been eating together since the beginning of middle school.
In #4, Mary Anne says that the table where they sit is round with only four chairs, and Jo DeFord takes Mary Anne's place there while she and Kristy aren't speaking. However, in book #2, the twins' first appearance, a girl named Lauren Hoffman is also sitting at their table, and Dawn sometimes sits with the four other girls after she comes to SMS.
In book #10, when the BSC begins eating together, the Shillaber twins are sitting with "the only set of boy twins in school."
The twins dress alike, at least sometimes. Their parents are divorced. They have an eighteen-month-old brother.
In #46, Miranda Shillaber is assigned to Mary Anne's research group along with Pete Black and Logan Bruno, but switches to another group due to a vendetta she's held against Pete Black since he broke the strap on her bra by snapping it during class.