As shown on the Snicket family tree, N. is the child of F. Snicket, and the (presumably middle, seeing as their name is listed second both on the tree and alphabetically) sibling of M and O. They are the cousin of G, H, I, Jacques, Kit and Lemony. This would make Beatrice Baudelaire II their first cousin once removed.
The main theory revolving F's children is that M and O are Miranda and Olivia Caliban, as they are both related, and as there are many interconnected VFD families, it would make sense for the Calibans and Snickets to be related. This would make N a member of the Caliban family, and would make Miranda's daughter, Friday Caliban, their niece.
Some theorize that N could be Vice Principal Nero, who works at Prufrock Preparatory School, which is (implicitly in book canon, canonically in the TV series) connected to VFD. Nero could fit into the Caliban theory, considering that all three names are connected to Italy.
Another theory is that M, on the family tree, is Monty Montgomery, which would make N a Montgomery, possibly Monty's sister. Outside of the initial and the fact that Monty has at least one sibling, there is not much evidence for this theory.
There is also speculation that O is Count Olaf, which would make N a count/ess, however this is unlikely considering that Olaf was in a romantic relationship with Kit, meaning they are probably not cousins.
|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|