I'm sorry this rude person is part of your life. Maybe make arrangements to see dad without her. Just the two of you get together for lunch? If you must see her, call her on it, when she says something, just tell her "That was rude and I don't appreciate it" then move on. Don't continue or argue with her, just state your displeasure. Maybe she will get the hint or not want to be around you so that you can see your dad alone. ~hugs~