I did wonder about the relationship, and if it might be a cousin or nephew or something like that. But prior to naming him, Henry said he had two daughters, Margaret McKelvey, and Mrs. Johnston. Don’t know why he didn’t put a first name for Mrs. Johnston, like he did for Margaret.
Henry did name his son Henry in addition to his son-in-law, Henry Johnston. I wondered if maybe the son-in-law could be Charles Henry or William Henry or something like that but I can’t find any Johnston,/Johnston, marriages that fit the bill. Unless it was common law. However, I don’t find any Henry Johnston, on the 1901 or 1911 censuses that might fit the bill either.