As shown from the Snicket family tree, G. is the child of D. Snicket and the (presumably elder, seeing as their name is listed first both on the tree and alphabetically) sibling of H and I. They are the grandchild of Chas. Snicket, the grand-nibling of A and B, the nibling of E and F, and the cousin of Jacques, Kit, Lemony, M, N and O. This would make Beatrice Baudelaire II their first cousin, once removed.
G is theorized to be Gregor Anwhistle, which would make his sibling I Ike Anwhistle. As many VFD families are interconnected, it would make sense that the Snickets and Anwhistles were related, and if D. was the parent of Gregor Anwhistle, it would make sense for them to be his mother, who was described by Ike's wife Josephine as having one eyebrow and one ear. H's identity would still be a mystery, though they are occasionally theorized to be Hector.
If G is Gregor, that would make them the second cousin of the Baudelaire family, and would mean that he is deceased.
|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|