As shown from the Snicket family tree, A. is the (presumably elder, seeing as their name is listed first both on the tree and alphabetically) sibling of B 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 A. on the key of the family tree is that their whereabouts are unknown.
|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|