(((onlymedid)))
Gosh....I wish that I did have some ideas of how to get through the upcoming visit!
I can only think of the following tips: Remind yourself that this visit is up to you. You control when you arrive and leave. You don't have to spend X amount of time with your step-mom. The amount of time that you spend with her is under your control.
Have you ever talked with your father about the abuse? I'm thinking not, which is probably why it brings up so much anxiety. Like, she's thinking you'll be there all of the time. Is that right?
Stay at a hotel/inn/motel, or even a friend's house. Don't dedicate all of your time to be spent with your parents. Maybe you can arrange to meet at the park, or something. Like, a middle ground, to be less reminder of the past? You can try asking your father to meet with you alone sometime. It's been a long time since you've been able to do that in life, and would like to have that experience again. Maybe that would bring you more strength.
I hope that my ideas help you get through your trip. Hopefully, others will add some helpful techniques that I didn't think of! Very best wishes to you, onlymedid!!