The 1880 census you mentioned doesn't list a relationship for any of these people, but it says Nettie was age 26 and Sarah was 78, which doesn't fit the other information for Sarah on the Family Search tree or make it possible for her to be in the age range to be Nettie's mother. It could be a census taker's mistake or bad info given, but it makes me a little suspicious that something could be off. There's also an entry on Family Search for Sarah in a different location under a different name for the 1880 census.
I don't know what the answer is, but I'd say to double check everything as much as you can and figure out if there's anything you can look for to fill in any blanks, like vital records, probate records, land records, mentions in the newspaper, etc.