Mcculloch of Thornhill Perth County Scotland

Catherine Graham married James McCulloch 1774 Port of Mentieth Scotland. Family legend has her connected to the Duke of Montrose. Don't think she was a daughter as some family members have claimed, but there is apparently some relationship which I have never proved. Her father may have been George Graham, and she may have has a brother named Robert. Would like info to further develop this ancestral line.