Thought I'd go ahead and list all the marriages I could find of the Stevenson line in case they come up in future research. Source - R.W. Saint Clair's Saint-Clairs of the Isles.
Douglas (c. 1610, Sir Robert Sinclair of Longformacus m. Margaret Douglas of Whittinghame. This Robert is brother to George, reputed progenator of the Stevenson line)
Earl of Crawford (often these titles are not last names, i.e. - Earl of Caithness, but it could be as the book listed no other name)