Bonnie is a character in The Vampire Diaries.
Bonnie has at least two older sisters. Mary, and another one who is in college. The phrasing 'one of Bonnie's sisters' makes it seem like more, as if it was Mary and one other sister, it would make sense to name her, or at least say 'Bonnie's other sister'. (The Struggle, P.366 double volume print).