John Martin - Sister called Martha who moved to Canada?

There is a possible sister of John Martin called Martha - his son Samuel James Martin mentions a Aunt Martha living in Canada twice in 1902 letters to his brother and sister-in-law from South Africa where he was serving as a soldier.
This is one possibility of such a sister Martha: Martha Balfour
For one, that woman's father was Robert Martin so could be John's father Robert Martin They seemingly have the same name.