It seems like you have made the decision to go, and are trying to figure out how to handle it, rather than deciding to go or not. Is that correct?
Like others, I think there is good cause to re-consider the decision to go.
You list only one possible gain by going: dinner and a movie.
These are some of the possible risks to plan for: being triggered, your step-dad being an asshole and your mother being silent again, being stranded with no way home, worsening of PTSD symptoms, delaying recovery, and/or being pulled further into dysfunctional family system.
Trigger planning is key to recovery in PTSD. If you choose to expose yourself to possible triggers, that is not always bad, if you are prepared and ready to dive into what trauma and symptoms could be triggered.
You know who they are and how they can be. You appripriatly expect there to be "unacceptable" behaviors or statments made. You can't change them. The only person you control is you. None of their behavior or abusive actions are your fault. Management of your PTSD symptoms is up to you. Fair or not, it is where things are at. If you leave it up to them, you will most likely walk away very disappointed, at best.
As for a trigger plan for your time with them, these are some bungs I do when I choose to be around family:
1.) limit the time - set clear boundaries and expectations of the amount of time I will spend with family
2.) If I rely on family to take me home, I have a backup plan to get home if I need to leave and/or they choose not to take me home in time. Since you and your fiancé are choosing the dinner and movie, maybe the location needs to be near a bus route. Maybe a friend needs to be on speed dial to come get you. This is partly about being able to actually leave, and it also gives your brain a way of knowing there is an ultimate escape. Triggers are more triggery when we believe there is no escape.
3.) bring things to comfort or ground if symptoms hit. I bring frozen water bottle, strong gum, photos of places that make me feel safe (like a travel photo of a beach), cards with positive statments on them (like "I don't have to please my family anymore.") I take time to review these prior to meeting with family and even during breaks when I go outside for fresh air or to the bathroom.
That is just a start of a list of possible things to add to your own trigger plan for your time with them.
Remember, you were helpless before during past trauma, but you don't have to be helpless anymore and let things just "play out." You can't control your mother or step-father, but you do have some signficant choices over who you will allow into your life and under what circumstances. You also do have options to mitigate any possible ways you could be triggered.