One fly in the ointment here...I thought Jane would fit so well with being your Alexander's sister. Both born about the same time in Alness, mother Christina...however, not getting them to connect that way.
This is your Alexander in 1851?
Donald Mckay 48 wood sawyer b. Kiltearn, Ross
Christy Mckay 38 b. Dingwell, Ross
Kenneth Mckay 15
Robert Mckay 13
Alexander Mckay 9 b. Alness, Ross
Margaret Mckay 7
Duncan Mckay 3
Donald Mckay 9 Months
Address: Bridgend, Rosskeen, R & C
This looks to be gravestone details for Donald and wife Christina. She looks to have died in 1872. Therefore, can't be the same as the Christina with daughter Jane in the 1881 census.
www.findagrave.com/memorial/157309037/christina-mackayI think this may be Jane with family in 1851:
Angus Mckay 50 cattle herd b. Kincardine, Ross
Christy Mckay 40 b. Kincardine, Ross
Mary Mckay 15
David Mckay 11
Jane Mckay 8 b. Alness, Ross
John Mckay 5
Isabella Mckay 3
Thomas Mckay 9 Months
William Manson 39
Kenneth Munro 22
Charles Sellar 17
Address: Ardross Easter, Rosskeen, R & C
Marriage and death certs will hopefully help to make things clearer...

Monica