Do you think Mrs. Saville, Robert’s sister, ever received those letters Shelly's Frankensrein?

Well, this question is open to interpretation.  The
literary purpose for those letters was to provide an outer frame for Shelley's novel
Frankenstein.  The character of Robert Walton provides both an audience and validity for
Victor Frankenstein's story.  The letters to his sister provide information about Robert
before we meet Victor and show us the parallels in their
character.

This being said, there is no real literary
reason the letters would have to be received by  Mrs. Saville as she is essentially a
VERY MINOR character. She does ever enter the story again.  However, there is no reason
they would not be delivered.  The letters would have traveled from boat to boat and port
to port, so certainly the timing of their delivery would be uncertain.  Some letters may
be lost; some may arrive out of order.  The idea that Robert is wanting to share his
experiences is important, so I would like to assume that his attempt was
successful.