Any reason you can’t use your words, OP?

Ugh. My now dead mom was a narc to her very core. Attempted and/or ruined every single family gathering and made it all about herself.

Is this HER family and relatives coming in for a milestone event?

She can sit in the hotel room alone for the entire weekend. Please keep to your plans and do not carve out one second for her. In this situation my mom would end up having “a health emergency” before/during/after big event with urgent phone calls, messages and maybe even transport by ambulance (I’m not kidding). Block her calls.

I’m in therapy by the way.

Your answer should be no way in hell are you running another special occasion, mom!
Trust me, FIL is the same horrible narc as your mom. Ruins everything.

Twist the knife.

“We’re celebrating/commemorating X and don’t want our focus taken away from that important milestone. Such achievements!

We’ll see you some other time.”

“No, we can’t do that day. We can do X or Y. Let me know if one of those works for you. If not, we look forward to seeing you at Thanksgiving!”

Stop. Dancing. To. Her. Tune.

Is she coming to you?
Is she seeing anyone else during this time?
I’m guessing she has other 1:1 meetings planned.
If she doesn’t, tell her to come another time.

Tell her she needs to be sitting by her phone at that time so she doesn’t miss the Newspaper reporter calling to do their big story on how special she is.

Or tell her Elvis came back to life for one day only and wants to spend his day with her alone.

No mom sorry, we are busy that day.

Wait so she is traveling there exclusively to see the two of you? I would tell her to cancel and come visit you in your home town when you are not busy with the event. But no, do not make this work.

"Sorry, that won't work for us. You could have picked a less busy time."

Do not go.

Offer a different time.

"This doesn't work for us."

Just say no.

We're hosting a milestone family/friends event. People are traveling from out of town. Some people are coming a day early or late to make an extended visit and attend secondary get-togethers. Some people are making a tourism vacation and just coming to the event and then moving on. Great.

One person, my mother, is staying at a hotel a substantial distance from the event, and not coming to the event (health reasons), and wants to have a separate 1:1 meetup with me and kid, same day as the event. We're expected to ditch everyone else and give up our rest time to go see her for this specific time window she selected.

Ugh.