Keep in mind that "Sally" or "Sallie" is often the nickname for Sarah. Am inclined to believe the mother's record and wish you great success in locating her information.
We have a Susan S Bigbee in our family b. 7/13/1874. We have heard that her middle name is Samarismith but have no proof. She was known by the name "Aunt Dink" all her life. We have no idea where that name came from. She was the daughter of Wm Thomas and Mary Ann (Lankford) Bigbee. He was born in TN and it is believed she was born in SC. WT was the son of John "Jack" Strother Bigbee b. abt 1818 Robertson Co.,TN.(moved to Fannin/Grayson Co., TX in the 1850's.)He ultimately had three wives and a total of 18 children, although not all survived childhood. He had an uncle of the same identical name(John Strother Bigbee) born in 1795, who was the brother of Jack's dad, Archibald. His family line moved to Missouri and then on west. I have read with great interest your search for Suse, Susie or Susan and understand perfectly what you are going thru. WT also had a sister (he named his dau after her)who went by Susan Bigby (your spelling) and her first husband was a Josephus/Joseph Blagg -----we are now searching for the second husband. We have a picture of one of their daughters--Mary Bell "Molly" Blagg Baldwin and she appears a beautiful indian maiden. We have been told there was indian blood in the family but have not been able to pinpoint the connection.
There are three or four spellings of Bigby, Bigbie, Bigbee and Bigbea(German spelling and believed to be unrelated initially) as I am sure you have found. I am wondering if you are kin to the Rev Thomas Bigby out of SC whose kin moved into Ga, Al, MS and west? I have access to quite a bit of information on family back to 1600's. Would like to keep in touch because I feel when you find your info it may tie into ours. Best Wishes and happy hunting, Bonnie