Christina 1 gets married on 2nd June 1888 to William Cooper. Funnily enough her brother William gets married on the same day.
He doesn't by any chance marry a Margaret/Maggie, does he? No, I see it looks from the census as if he married a Jane.
We have several choices for Aunt Maggie. If she was married, then she could have married Christina #1's brothers William, James, John or Alexander
Or, she could have married Christina #2's brothers Theodore, James or John
Here's the 1851 census showing the brothers of Christina #1 who had left home by 1861:
Invertherny, Auchterless, Aberdeenshire
John Simpson, 36, Head, born Forgue, Aberdeenshire. Wright
Jane Simpson, 30, Wife, born Auchterless, Aberdeenshire
William Simpson, 14, Son, born Turriff, Aberdeenshire, Scholar
James Simpson, 11, Son, born Turriff, Aberdeenshire, Scholar
John Simpson, 9, Son, born Auchterless, Aberdeenshire, Scholar
Elisabeth Simpson, 8, Daughter, born Auchterless, Aberdeenshire, Scholar
Helen Simpson, 5, Daughter, born Auchterless, Aberdeenshire
Jane Simpson, 3, Daughter, born Auchterless, Aberdeenshire
Christian Simpson, 3 Mo, Daughter, born Auchterless, Aberdeenshire
Alexander Hendry, 20, Apprentice, born Auchterless, Aberdeenshire, Wright App
Can't find anyone in the Christina #2 family in 1871 to see if there were more sons. I can only find Christina herself as a servant in Auchterless.
No time to look further at the moment.