It's a MIL one - stop me before I say something I'll regret

I would be VERY pissed off in your shoes. I'm not sure I would be that confident DH would accurately convey your upset about it either, if he's a laid-back people-pleaser who doesn't like to upset his mother.
I would back right off and be very cool with her (but that's me). And if she ever had the gall to ask me why, I'd tell her.

I think OP, after reading your updates, you and DH need to reduce the importance of her presence. She is probably feeling super important having all these people wanting her here and there and deeming which event suits her most. I would step back and never organise anything again.

I think she will pretend the car has broken down or traffic is bad, and secretly attend some event after the play and not return. No way based on how you have detailed her obsession with her grandson, will she leave right after the play to drive 1.5 hours back.

You are all better off without rude grandma in the picture. She needs to take a backseat. The star of your show is DD and she’s obviously has two great parents, she doesn’t need anyone who is half-arsed in her life. She’s way more valuable than that.

I hope your DD has a great time meeting Santa!

* its a 2 night stay because DH works in retail and can’t get more time off in Dec.*

oh sorry op I didn’t read it properly obvs!

With all your updates op I have to say she sounds totally batshit and I feel sorry for all of you, this is a very unhealthy way to grandparent, I even feel bad for your SIL being constantly undermined, how awful.

Next year in Copenhagen sounds fantastic.

I don't think this is necessarily the case.

Might be MIL's own DD said it was happening without being specific when, asked if she wanted a ticket, agreed and arranged it all and then found out it was on the day of the trip.

Honestly, if the trip has been booked for them (which sounds kind) but isn't to their taste, maybe they don't want to go and are glad of a way out. I'm not sure 1 night with the activities planned is really a fair swap for the holidays OP says they've taken her on.

A day trip to centre parcs? Is that a thing? It’s not really a holiday is it?

On balance though I think your IL’s should stick to their original plan for the sake of keeping the peace. Your sil can record it or buy the dvd.

And I agree with a pp - where is the anger at FIL? Why is it just aimed at MIL?

Reading your other posts, your MIL sounds obsessed with her DGS and I feel sorry for SIL.

My PIL are similarly obsessed with my firstborn (her first grandchild). When he was born, they tried to move in with us so they could see him everyday. They're still crazy about him and less so with my second-born. My DH is one of two (he has an older brother) and BIL is child-free by choice. My MIL constantly expresses how disappointed she was and how long she waited to be a grandma. Sometimes, I wish BIL had chosen to have children and taken some of the pressure off from us.

Honestly OP if I were you I'd be glad she's not trying to have that horrendous smothering relationship with your child! But I can understand you don't want her to feel rejected and left out when you're all together.

This will have been said, I have only read OP posts. Tired and sick children here.

But I would take it as your out.

Oh that's such a shame MIL. That change doesn't work for us due to planned activities so x and y are coming instead. See you soon.

I think I'd be very tempted to send something like this back, emphasising that you and DH know that this trip was in her diary for months and the nativity (the second one that she will be rocking up for) could only have been in the diary for a few weeks and that it's just not on to do something like this.

Or your DH should. Probably better coming from her own son stating how disappointed he is that his parents are playing favourites because if he has noticed, it must be as plain as the nose on their faces to everyone else.

She has presumably just got the date for the Nativity Play and decided she can (just about) do both things.

Is the Nativity only on once?
Can SiL definitely get enough tickets for GPs to attend (my adored grandchild has a starring role in their first - and possibly only - Nativity but they are strictly limited to 2 tickets per child so only parents are going)?
Could you amend your trip by one day?

Enjoy your trip whatever happens.

I can’t understand people saying you can’t miss a nativity play etc. I hated them. Thought they were crap. Same kids being given star roles every year.

YANBU OP. I’d be fuming. Centre parc isn’t exactly cheap.

Yes, I'd say even more valuable because statistically, aunt and cousin will be around longer.

MIL could use some therapy. I'd just stop making plans with her.

If it’s only 1.5 hours away can you not rearrange Santa for the afternoon of the first day? Set off at 9 you’ll be there by 10am, book it for around 1pm? They can then be at Santa visit with one grandchild whilst being able to go to nativity with other grandchild.

I've just scanned back to see what it was you were wanting to get out of this thread. Yes, do reply and tell her to forget the whole trip. I think if she comes and leaves, it prey on your mind and ruin the trip, if they come and stay she will undoubtedly make some mention of the nativity and it will ruin the trip. Better that they don't come at all!

Having caught up with all your updates OP I agree with this poster. You are much better off.

She sounds like an absolute horror. I’d pull back as much as possible tbh. It will be very interesting to see how she is with the new baby, that might be the thing that brings it all to a head. I doubt your SIL will let her show blatant favouritism between siblings, should it happen.

OP, I’ve read all your posts now and clearly your MIL has issues.

Thing is, I bet your SIL is as fed up with her as you are, so I would suggest getting your sil on side - I would in fact mention to her that it’s a shame that MIL has essentially blown off part of the holiday to be with DGS.

I think rather than wanting MIL to have more to do with your DD, I bet your SIL would prefer her to be less possessive of her DS.

If your SIL is saying that, you may actually have reason to be thankful that she is less engaged with your family...

Regarding this event, I would just let it go. As you say, it will make no difference to your DD and it is the relationship between them that matters. If your DD would have been upset, it would have been different. For your own benefit in the future, I would back off making such arrangements and let MIL do the work herself. Your DD will be fine if they end up only having a distant relationship.

I actually think you're really lucky that your MIL doesn't smother your DD in the way she does her DGS. Be thankful, not jealous.

Let her get on with whatever she wants to do and let it all wash over you. You'll be happier in the long run, I promise.