Janet or Jessie just disappears after 1871.
Janet/Jessie Thomson married William Milne, apparently on 12 Feb 1875 at Tulliallan (according to the birth records of two of their children), although I can find no record of the marriage on SP. The William Milne, grandson, who appears in the 1881 census with William Thomson and Margaret Cowie is their son.
This is the family in 1881, 1891 and 1901 in Buckie:
1881
William Milne, head, fisherman, 39, Buckie, Banffshire (think the age may actually be 29)
Jessie Milne, wife, 36, Buckie, Banffshire
Jane Milne, daughter, 3, Buckie, Banffshire
1891
William Milne, head, fisherman, 38, Buckie, Banffshire
Jessie Milne, wife, 44, Buckie, Banffshire,
Jane Milne, daughter, 14, Buckie, Banffshire
Williamina, daughter, 9, Buckie, Banffshire
Helen A Milne, daughter, 5, Buckie, Banffshire
1901
William Milne, head, fisherman, 48, Buckie, Banffshire
Jessie Milne, wife, 50, Buckie, Banffshire (age might be 56)
Jeannie Milne, daughter, 22, Buckie, Banffshire
Helen A Milne, daughter, 14, Buckie, Banffshire
William Smith, nephew, 3, Buckie, Banffshire
Alexander Milne, nephew, 1 mo, Buckie, Banffshire
Jessie Milne (ms Thomson) died in 1927 in Newhills, Aberdeenshire.
Ruth