Have you looked at the actual OPR entry? sometimes a parent is mentioned at the time of marriage, if there was a conscientious clerk involved
It doesn't look as if either of them lived until 1855 either

Is this the extended family in 1841?
Piece: SCT1841/185 Place: Cruden -Aberdeenshire Enumeration District: 1
Civil Parish: Cruden Ecclesiastical Parish, Village or Island: -
Folio: 1 Page: 10
Address:
Hill Gask
Surname First name(s) Sex Age Occupation Where Born Remarks
BROOK George M 30 Farmer Aberdeenshire
BROOK Jean F 30 Aberdeenshire
BROOK Elisabeth F 70 Aberdeenshire
BROOK Elisabeth F 10 Aberdeenshire
BROOK Isabella F 6 Aberdeenshire
BROOK Alexander M 8 Aberdeenshire
BROOK George M 4 Aberdeenshire
BROOK Thomas M 18m Aberdeenshire
SIMPSON Isabella F 20 Female Servant Aberdeenshire
SIMPSON Robert M 20 Ag. Lab. Aberdeenshire
BROOK Elespet F 35 Aberdeenshire Lodger
by 1851 'granny' is no longer there:
Hill Of Gask
Surname First name(s) Rel Status Sex Age Occupation Where Born Remarks
BROOK George Head M M 46 Farmer 70 Acres Aberdeenshire - Cruden
BROOK Jean Wife M F 43 Aberdeenshire - Longside
BROOK Elisabeth Dau U F 20 Employed At Home Aberdeenshire - Cruden
BROOK Alexander Son U M 18 Employed On Farm Aberdeenshire - Cruden
BROOK Isabella Dau U F 16 Emploued At Home Aberdeenshire - Cruden
BROOK George Son - M 14 Scholar Aberdeenshire - Cruden
BROOK Thomas Son - M 11 Aberdeenshire - Cruden
BROOK Nathaniel Son - M 9 Scholar Aberdeenshire - Cruden
Page: 170/10 BROOK James Son - M 6 Scholar Aberdeenshire - Cruden
BROOK John Son - M 3 Aberdeenshire - Cruden
The naming pattern of Childen can often help work out parents' names, though the IGI only seem to have George 1805 belonging to the couple.