Does anyone have some recommendations on what I can talk about and how to maintain my cool. I'm getting anxious. I am not doing this because I want to. I am doing this because my sister (the golden child) has made it a stipulation. I think that my mother thinks this means things will go back to normal. I plan to have as little contact with her as possible after the wedding. We'll just see each other at some family events and that's all. She'll be very upset when she realizes that. Should I say anything about that today? I feel like I need a plan.
I hope things went ok

My family is allergic to intimacy. Anything that feels like closeness leads to drama.
Like Methuen mentioned, I keep things as neutral as possible. You're smart to meet in a public place for lunch with a socially acceptable time limit.
I've found over the years it helps to have catch phrases that can prevent conversations from getting too real.
Real --> intimacy.
Intimacy --> fear.
Fear --> anger.
My family has very little tolerance for anything real. I try to use facial expressions more than I do with other people because non-verbal validation seems to help keep them more evenly keeled. I have a theory that if I appear distant or remote, or have a muted facial expression, they feel the need to stir the pot and get a reaction. Then, if I have a reaction that is remotely emotional, they withdraw. They don't want intimacy, and they don't like messy emotions, but they don't like grey rock either.
It's been a process trying to figure out how to feel emotionally safe in relationship with them. I don't know if any of this will be helpful because our circumstances are often so specific but maybe there's something in how I try to navigate my post-estrangement relationship that is useful for you?
I wish people like us could have meaningful conversations with family but I'm learning that isn't in the cards for me. If I achieve any kind of closeness there is often a push back to reset things to status quo. And I find that swing of the pendulum to be quite painful so I don't pursue intimacy even if they do something that might seem like a hint of intimacy.
I'm learning to spend 90 minutes saying almost nothing:
"He's doing great. How about ______."
"That's nice."
"That's interesting."
"Oh?"
"Huh."
"Wow."
"And then what?"
"Excuse me for a moment."
If things take a turn toward hot button topics, even if it's just in that direction, I have a handful of helper phrases:
"Let me just respond to this real quick."
"Did anyone see where the bathrooms are?"
"One sec. Where's the waiter ..."
If they ask me to do something:
"Let me take some time to think about it."
"Maybe next time."
"You've given me lots to think about."
"I'll look at our calendars when I get home."
Underneath all this my emotions are usually ping ponging around inside. I try to pay attention to my breathing since I'm prone to muscle spasms when I visit them.
My family is covertly aggressive unless alcohol is involved. I learned pretty quick I cannot drink alcohol around them, not even a glass of wine because I need my full faculties. When alcohol is involved, they are quick to trigger. The last time I had a glass of wine with my family, my father became furious about me taking too long to get ready for school 40 years ago.
I'm glad you have a partner supporting you. I don't know if I could do this without the help of my husband. Sometimes I think he sees them more clearly and follows what's happening when we're in the thick of it. I can feel some of my old survival strategies at work, making it hard to fully take in who is doing what.