Eighteen years is a long time to delay vengeance. Let's substitute plain old lust.
GILFy granny Greta was madly in love with her studly hubby Hank, who died much too young. As her son Scott matures, she sees him as the spitting image of Hank, thus her seduction of him. He accepts their taboo relationship awhile, then falls for and marries Barbi, who bears twins Tom and Toni. Tom grows into another replica of Hank, while Toni looks just like her mother Barbi.
Because this is LIT, Greta and Barbi both gave birth at 16 and are tennis-toned. Just saying.
When Tom and Toni turn 18, GILFy granny Greta, now a lithe 50, once again sees her Hank reincarnated. Powered by undying lust, she seduces Tom. Has she also been getting some from Scott all these years? Once Tom learns the joy of incest, do his thoughts turn to his sister and their lookalike MILFy mom? Once Toni learns the joy of brother-fucking, do her thoughts turn to their lookalike dad?
Do their carryings-on alarm the livestock?