I think maybe you are missing or perhaps mixing up the generations. The marriage certificate of George Sweet and Mable Dykeman can be seen at the NB Archives site. George's parents are listed as Charles Edward Sweet and Mary Neely
http://archives.gnb.ca/Search/VISSE/141B7.aspx?culture=en-CA...George and Mable are the parents of Charles Edward Sweet according to his birth record which can also be found online at the N B Archive site. He was born 1905 in West St. John, NB. His grandparents were Charles Edward Sweet and Mary Neely.
You can see George age 18, living with his parents Charles and Mary in the 1901 Census
https://familysearch.org/ark:/61903/1:1:KHKC-15KAnd Charles with his parents George and Mable at
https://familysearch.org/ark:/61903/1:1:QV95-Y3Y5I can't explain the error on his marriage certificate; why he would have listed his grandmother as his mother. But the majority of records seem to prove that this was a mistake.