Meredith Grey (Ellen Pompeo) makes out with Owen's sister's ex husband Nathan Riggs (Martin Henderson) on Grey's Anatomy season 12, episode 23 (At Last).
Grey's Anatomy semi AU. Meredith & Finn are in a relationship until she cheats. Derek & Meredith begin their relationship, until he cheats with Rose. In the end, Meredith is alone and has no one.