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.

Lauder

Swinton

Home

Foulis

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)

Stewart

McMath

Boyd

Scott

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)

Balfour

Strachan

Dundas

Menzies

Lockhard

Bogle

Kerr

Newman (1810's)

Mercer (1820)