I agree with pp actually, 'talking to your Dad about your SS' will put him in an awkward situation.
I agree the letter was mean-spirited, and especially at christmas, but I am friends with a girl who is in the SS's shoes. SHe's not a bad person at all but she's probably done some bonkers things like this. I know she's written a letter like this to her stepMum. She just feels like she's on the outside looking at her family and knocking on the window and she can't get IN.
I know it's easy for me to be objective and say to her that even in a FAMILY you can't demand loyalty and love. You just have to act like a person who deserves it and hope it comes.
Maybe your Dad could go to lunch with just his daughters??? It means nothing to you. You can afford to be gracious and not care! because it#s nothing to you and you know you have a good relationship with him. It might smooth her ruffled feathers. SHe's probably not actually trying to be a bitch. She's probably upset.