Frankenstein is written in the form of a frame story that starts with Captain Robert Walton writing letters to his sister. It takes place at an unspecified time in the 18th century, as the letters' dates are given as "17 --".

Answer this question based on the passage: to whom does the narrator write in frankenstein? Let's have some stream of consciousness first.
This is the relevant information: Frankenstein is written in the form of a frame story that starts with Captain Robert Walton writing letters to his sister. So, the answer is his sister.