In case brother John DID refer to brother-in-law John, I decided to check for deaths before embarking on a costly census trawl. There were a few likely entries but since John hadn’t appeared on any of the censuses, I chose one of the infant deaths first and sure enough there he was:
Poor little tyke only lived 12 days – cause unknown.
I think it’s safe to say that the John present at Alexander BUCHAN’s death is his sister Jessie’s (born 1860) husband. They had married back in Peterhead and emigrated with Jessie’s parents and siblings.