Hey, welcome back
Navigating holidays plus disordered family members can be so stressful, especially when there's no guarantee, because everyone lives close to each other, that things will go exactly how you are hoping and planning.
If I'm tracking with you, it's not that you've fully cut off your mom and have vowed never to see her again... it's more that
right now you aren't up for seeing her. Is that accurate?
Do you feel like the other family members you want to see (maternal grandparents) would do anything like "secretly inviting her to make you two reconcile", or, if you choose to see them, would it be pretty straightforward -- you know if you go, it'll just be them, no funny business?
One approach to these holidays could be to make your plans based on what you want, assuming you can trust other family members to be straight with you about it. (I did once tell my mom, who does not have BPD, that if she invited her BPD friend to our family Christmas, I wouldn't be able to make it -- and fortunately, my mom was honest with me and didn't do the move of saying "of course she won't be there" and then sneakily inviting her).
It's true, you might run into your mom. I wonder if you can reflect on the likelihood of that happening (is it like a 5% chance? or like a 95% chance?), and then, most importantly, reflect on how much you have in the tank to handle that. Getting that information might help you make a decision. For example, if you find that there's a low-but-possible chance of running into your mom (say 10%), but that you're running on empty and do not have the resources to handle it, that's good to know. Same if there's a high likelihood of seeing her, but you feel pretty grounded and capable, and have some stock phrases you could say. Important info.
Ultimately, it's up to you, and your energy, and your bandwidth. I'm thinking that checking in with yourself -- really seeing what you are OK with, what you can handle -- will be the most deciding factor, not what anyone else is or isn't doing. that's one of the strengths of this group -- nobody has to be the a-hole; we can move away from "you're wrong and she's right"/"you're right and she's wrong" and towards more healing and centering options, like caring for yourself and knowing your own strengths and limitations
...
Speaking of bandwidth, do you feel like you're in a position where you could let your mom know something like:
"Hi Mom, I already have Thanksgiving plans this year, FYI -- I do currently have Christmas and New Years open, so if you want to grab coffee around that time, let me know"
(that would depend on if you're moving towards communicating with your mom again).
Or, it might need to look shorter, like:
"Hi Mom, thanks for asking -- I already have Thanksgiving plans. Cheers; KiaraBaneTME" where you don't suggest an alternative further down the road.
Keeping things
Brief, Informative, Friendly, and Firm can help communications stay on track.
Or, a further option would be not to communicate with her about your plans at all, to honor the fact that right now --
for this moment -- you aren't up for seeing her, to decide for yourself if you can handle accidentally running in to her or not.
Hard stuff to navigate. I get it about the "small town" feel; my H's kids' mom and stepdad live <1 mile from us, so it's a non-negligible chance that we run into one of them by surprise around town. It can really rattle you if you are overwhelmed, so I'm glad you're here getting some support.