She and her family worked with the underground railroad, but when her husband Hiram betrayed their activities to the slave owning Colonel Burns, she fled with the runaway slave Virgil to Canada, meeting Abraham Lincoln along the way, who offered that she cover her conspicuous hair with his stovepipe top hat. Later, she legally divorced Hiram (also getting one of his shoes in the divorce settlement although he keeps the laces) and married Virgil, who had proposed to her on a bridge marking the border between the United States and Canada. Since Virgil had no last name he took the surname "Simpson." Mabel and Virgil had one known child, Abraham, whom they presumably named after Abraham Lincoln and who himself is the presumed namesake of Abe "Grampa" Simpson, Mabel's great-great-grandchild. She is identical to Marge, which is strange, because Marge is a Simpson by marriage, so it doesn't make much sense.
Mabel is a great-great-great-great-grandmother of Bart, Lisa and Maggie, as well as Grandpa Van Houten, which will make them fifth cousins.