 In 1735, St Clair bought Rosslyn Castle, which was later inherited by the male heirs of his sisters.On the death of his older brother John St Clair in 1750, he succeeded de jure as Lord Sinclair, but never assumed the title, preferring to retain his seat in the Commons.Around 1745, he married Janet Dalrymple, the youngest daughter of Sir David Dalrymple, 1st Baronet and widow of Sir John Baird, 2nd Baronet.