Who is r. walton and why is he writing to his sister

He is an american author who was writing to his sister who lived in brittan he missed her very much- Hope this helps :D

If we're talking about  in Mary Shelley's "Frankenstein" - Then I have the answer. 
                                                                                                                               Robert Walton is an explorer. The leader of an expedition to the North Pole. He begins the story through letters he writes to his sister, Mrs. Margaret Saville. He is attempting to sail to other regions of the world that mankind has never seen.