This looks like John and Helen's banns/marriage:
JOHN DOUGLAS and HELEN/NELLY DOUGLAS 27 APR 1821 Farr/ 27 APR 1821 Lairg, Sutherland
Helen still shows as married in 1851 so husband John is still alive then and probably working away from home. Hopefully they both made it past the start of official registration in 1855 to let you confirm their parents. This will help in particular in respect of Helen and her English birth.
In respect of John, I think this may be his birth and also some siblings. To parents Thomas Douglas and OLIVER YOUNG:
1. WILLIAM DOUGLESSBirth: 07 DEC 1798 Yetholm, Roxburgh
2. JOHN DOUGLASS Birth: 26 JUL 1800 Yetholm, Roxburgh
3. THOMAS DOUGLASS Birth: 04 OCT 1802 Yetholm, Roxburgh
4. JAMES DOUGLASS Christening: 21 MAY 1805 Yetholm, Roxburgh
In 1851 in Cilbrig, where you have Isabella being born, shows this family:
James Douglas 45, shepherd, b. Huetina, Northumberland
Cathrine Douglas 37, b. Assynt, Sutherland
Elisa Douglas 12
Alexander Douglas 11
Oliver Douglas 10, daughter
Cathrine Douglas 9
Thomas Douglas 7
Jean Douglas 4
Oliver Douglas 80, mother, Formerly Shepherd Wife, b. Yetholm
Donald McKay 29, visitor
Adam Munro 25, visitor
Mary Sutherland 19, visitor
Isabella McKenzie 16, servant
John Sutherland 50, visitor
William Lucy 27 , visitor
Address: Clibrig, Farr
Monica