Hello, you fit right in here

I identify so much with the nausea. I've done a lot of therapy and come a long way but I still seem to get back spasms and migraines when I visit my family, and one time got a case of shingles an hour before driving to see them. That's without any active conflict.
One of the lessons I've learned here is to use BIFF statements (brief, informative, friendly, firm). Less is more. You want a small target.
Often the hard thing is managing the physical and emotional sensations that come up when we change behaviors. It will probably be tough to not respond, although that's a perfectly reasonable option.
If you do text, keep it short. It may take you time to work up to other changed behaviors so it's really about what you feel strong enough to tolerate.
"I'm taking a time out this week. Maybe another time when I'm feeling better."
"I'll get back to you on that when I have a better handle on things."
"How about never."
Just kidding on the last one.
Buy some time for yourself. It's ok to be non-committal and vague. A lot of us grew up in families with no boundaries and oversharing was part of how the family system maintained control. However, it's perfectly ok to hold space for yourself and not let people rush you into things you don't want to do.
They'll definitely freak out because you're changing behaviors and that will scare them (losing control). But you'll get a bit of time to get centered and settle in for this important new chapter.