I am thinking that there are possible oedipal feelings that Robert Walton has for his sister. The way his refers to her at the ends of his letters, especially the 2nd letter, is quite affectionate.

"Continue for the present to write to me by every opportunity: I may receive your letters on some occasions when I need them most to support my spirits. I love you very tenderly. Remember me with affection, should you never hear from me again.

Your affectionate brother,
Robert Walton"

Maybe it's just the times, but I'm not so sure people refer to their sisters in such a manner.