As shown from the Snicket family tree, B. is the (presumably middle, seeing as their name is listed second both on the tree and alphabetically) sibling of A. and Chas. Snicket. This would make E. Snicket their niece, and her siblings D. and F. their other niblings. This also makes Jacques, Kit and Lemony their grand-nephews and niece, and Beatrice Baudelaire II their great-grandniece. Their other grand-niblings are labelled G, H, I, M, N and O.
All that is said about B. on the key of the family tree is that their whereabouts are uninteresting.
|A. Snicket||B. Snicket||Chas. Snicket|
|D. Snicket||E. Snicket †||Jacob Snicket †||F. Snicket|
|G. Snicket||H. Snicket||I. Snicket||Jacques Snicket †||Kit Snicket †||Lemony Snicket||M. Snicket||N. Snicket||O. Snicket|
|Beatrice Baudelaire II|