Ross appeared in the big-screen fairy tale as the mother of a younger version of Stewart's Snow White character.
Both Sanders and Stewart apologized publicly for their affair and insisted it was over.
"This was a long time coming," UsMagazine.com quoted a source close to Ross and Sanders as saying about Ross' divorce filing.
The insider said Sanders "did want to try to repair the relationship and didn't want a divorce." However, a second person with knowledge of the situation told the magazine Ross was too upset to forgive and forget.
The couple have two children together -- Skyla, 7, and Tennyson, 5.
Stewart and her longtime beau, actor Robert Pattinson, broke up briefly after the affair, but have reconciled.