As shown from the Snicket family tree, D. is the child of Chas. Snicket, and the (presumably elder, seeing as their name is listed first both on the tree and alphabetically) sibling of E. Snicket- the mother of Jacques, Kit and Lemony- and F. Snicket- the parent of M, N and O. This would make Beatrice Baudelaire II their grandniece.
D's children are theorized to be Gregor, H (possibly Hector), and 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 Ike Anwhistle, it would make sense for them to be his mother, who was described by his wife Josephine as having one eyebrow and one ear. This could make unibrows a family trait, as that was a physical identifier that Jacques also shared.
If D. is the woman with one eyebrow and one ear, it's extremely likely that she was Ishmael's student who lost her ear in an accident in her grandfather's laboratory. He says that after she was orphaned, she was living with her siblings under the care of a violent drunkard and murderer. She was concerned that her guardian may be poisoning her tea, which, instead of drinking, she would dump into a houseplant. She went to Ishmael with concerns about being poisoned, which somehow led to him arriving on The Island.
|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|
- If we assume that D. Snicket is the girl that Ishmael talks about, D. is grandmother-age now, so to be her teacher Ishmael would have to be at least about 20 or 30 years older than her. Therefore Ishmael is sort-of between the grandparent and great-grandparent generation.