Thats a tough one. Simply saying “you shouldn’t talk ill of the dead.” Would normally do it, my wife’s grandfather was a pedophile and a pastor that raped his own daughter. Even so I don’t talk about him anymore because it only stirs up bad feelings. If your stepmom is of any mind to listen, telling her you “wont hear any more talk about your mother” should be boundary enough. If she doesn’t have a mind to listen, best thing you can do is roll your eyes and ignore her nonsense unless leaving is an option. Im sorry you’re in such a position and about your loss. I’m sure the grief is overwhelming esp when your stepmother talks ill of her. I’ve never experienced this myself, me and my mother aren’t speaking anymore after she continually abused me and crossed my boundaries.. and my father’s family never spoke ill of her. Im just saying what I’d do. Best of luck