Sunny's name origin

Due to chapter 14 of the book The End Baudelaire family children are named after their acquintances who died. Violet would have been named Lemony, who was thought to be dead at that time, if she was born a boy. This fact made fans think about Sunny's name origin.

Theory 1: S. Theodora Markson

Because S. Theodora Markson's name is not fully known, it has been specualted that her named could be Sunny and Sunny Baudelaire was named after her, since S. Theodora Markson was Bertrand's chaperone. It is unknown if Markson is dead or alive. Also her name most likely stands for Solitude as Daniel Handler heavily implied in his interview.

Theory 2: Someone unknown at this point

The second and more possible theory is that Sunny was named after Baudelaires' deaceased friend unknown at tis point of time.

Sources:

Sunny named after S. Theodora Markson