Jean Ballantine, the lady whom Mary and Josiah are residing with in 1841, could be their aunt.
There's a Jean Gibson born 1788 in Kilbucho, Peeblesshire. Same birth year and birthplace listed in the 1851 census for Jean/Jane Ballantine.
Not sure if this John Gibson is connected to Mary and Josiah (or Jane/Jean Ballantine):
"Apprentice to, and eldest son of, Archibald Gibson, W.S. Born 1st February 1789.
Died 31st January 1879. Married (1) 21st July 1818, his cousin, Catherine, third daughter of John Dickson of Kilbucho, Peeblesshire, Advocate ; and (2) 10th April 1849, Mary Ann Mazyck, second daughter of Paul Weston, Physician, Charleston, South Carolina, U.S.A. Treasurer, 1862-70".
April 1849: John Gibson, a Writer to the Signet, married Mary Ann Mazyck (of Charleston, South Carolina), in Portobello, Edinburgh.