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

So she can still keep claiming that she "treats all her Grandchildren the same" when everyone knows its not true?

I agree with this. Let her have a taste of her own medicine. There's no way I'd let her off lightly with this - just like with children she needs to understand there are consequences for poor behaviour. She'd love you to brush it under the carpet so she can continue with her selfish, batshit behaviour and think everyone is OK with it.
You're a much nicer person than me @Bounty9 I'd be raging.

This.

She sounds like she's trying her best for both her grandkids. A nativity is a big thing to miss. Cut her some slack.

I wish my dc had a grandmother that cared enough to drive backwards n forwards like this.

She said she feels bad - she understands why we're upset but essentially it can't be helped.

"Can't be helped" tells you everything you need to know. She puts her wants first and always will. She had a golden chance to change her ways after the holiday, esp given your update on her own experience with her mother. She could change. She just doesn't want to. This would make me leave her to it.

"She said she feels bad - she understands why we're upset but essentially it can't be helped." - It's a shame your DH didn't challenge her and say 'but it can be helped mum can't it; by you honouring your agreement to come on the break that we've paid for'.

Why are you seeing her on Monday? Surely you have had a 'better offer' by now OP and it's far more important that you do your 'better offer' than to see her? 😉🤔

Same and I wouldn't ask after the nativity when she returns.
As for the posters saying mothers are naturally more close to their daughters children, the OP has done all she can to have a balance between parents and in laws. I dint have daughters so I sincerely hope that my DILs will be as thoughtful as you are when I become a GP

I get you OP
MIL was great, so helpful with DD and showed me how to be a great Mum BUT as soon as SIL had her child it was like a switch was flicked and DD was shunted aside. I had DS around the same time and I was expected to help SIL as well as we lived closer at that point than MIL. Then SIL got pg again very quickly and DD and DS were even less interesting to her, especially once they moved into a house MIL owned on the same street.
MIL likes people to need her, she's got a hero complex and my self sufficiency seems like an insult to her.

But she can disappoint her Grand daughter instead?

There is no doubt to give her the benefit of.

Please don't let her spoil your break with this ridiculous plan.

If she is that close to her grandson it is quite likely that he is expecting, and looking forward to them being there to watch and she doesn’t want to disappoint him, that is probably why she is sticking to her plan.

She said “it can’t be helped.” That would really annoy me. Yes it can be helped by her choosing to honour the original plans she made. Plus the boys parents can go to the nativity, and she went last year.

That must really grate that she thinks it can't be helped, because obviously it can be helped and she has made an active choice to disrespect her commitment to you and your child.

However, having read your updates she really is quite unhinged about this first grandson of hers and her inappropriate behaviour. Your SIL must find it all rather trying as well even though she is her daughter.

You're being very pragmatic about it all which is good of you. She will be the loser with all of this in the long run. She constantly shows you her true colours and you can only go forward making your own plans that don't involve her in things like this break in the future.

posted without reading all your updates OP, so I've deleted my post.

When they do leave to make the 1.5 hour journey to see DGS in his nativity, I honestly would barely look up from what I was doing. I would not make it easy with 'safe journey, see you at 5pm' or whatever.

It absolutely can be helped! MIL has accepted an expensive gift and is choosing to let you down because she has a better offer (in her opinion). Rude, entitled and appalling behaviour. If you accept an invitation you are committed, regardless of receiving a "better" offer.

I agree there is no point them going and wanting to be elsewhere, on that basis I honestly believe they shouldn't come at all.

Can you cancel their part of the trip and get a refund / invite others. It may be mid week but people often have holiday to use up at this time of year, you may be pleasantly surprised. It may also be possible to amend the booking to 3 - certainly worth enquiring.

Even if you can't get a refund / amend the booking / replace them, the trip will be far better if they don't come. The only whole day you have will be disrupted by their leaving / returning and then invariably telling you all about the nativity 🤦‍♀️

Don't put yourself in that position, enjoy the break as a family and DONT ever invite them again.

When you see her on Monday, I'd make it crystal clear that when she is with you and her son and your child, she is not to talk about the nativity or anything at all to do with SiL or their child or their family because that means that she isn't 100% present with you and enjoying the trip that you (and your DH) paid for for them. Not a word. Any forms of comparison between the grandchildren - off the table. Complaints about feeling tired due to the amount of driving - off the table, anything at all that could be connected to their decision to try to appease both sets of parents - off the table.

She and FiL are there to spend time with you and take part in the events for your daughter and I'd make it very clear that if she feels that she can't do that, then you don't want her there any more. FiL is welcome to stay, but her, not so much.

Would Pa In law go with you and leave MIL to her own plans? He sounds much more reasonable about everything.

100% this.

It can't be helped...remember that phrase. Exactly as she has said it to you because you just know it will come in very handy when she asks you to do something. "Oh...sorry MiL, it can't be helped."

It most certainly can be helped.

Anyway - at least the one thing that has come out of this situation is that your DH can now see how his own mother is playing favourites with her children and subsequent grandchildren so the gloves (so to speak) are off for anything that might happen in the future.

OP, I really admire how you've tried to foster a good relationship between DD and MIL. I think grandparents and the wider family circle in general can have such a hugely positive role in a child's upbringing.

That said, if DD is blatantly being passed over then I can see the whole thing becoming toxic for her instead.

Have you considered focusing your energies on SIL? Try to see her often (without MIL!) and encourage a close relationship between all the cousins?

As to the trip, I would be tempted to dis-invite MIL and either get my money back or give the tickets to friends. Polite visits over lunch or just a quick coffee might be the way to go in future.

Also, has anyone pointed out to her that she's repeating history with how her mother treated her grandchildren?

I think that’s really unhealthy and incredibly selfish of your MIL. She’s had her babies. A strong, loving relationship of course, but this sounds creepy.

My Mil has been threatening to die for around 10 years now (including at MY Mothers funeral) but theres no sign of it.
Obviously I don't actually wish her dead (honest) by the emotional blackmail is wearing, especially since she knows it doesn't work on me so she targets DH and The DC.
I hope you manage to have a nice break anyway OP and I might just arrange to be off somewhere on site having fun with my phone off when they are due back.

I would tell her on Monday she doesn't need to bother coming, because the clash IS avoidable – it's just her choosing to prioritise DGS. You also know that if she does come, she'll just bang on about him and the nativity the entire time and be too distracted to give your DD the attention she deserves for a change. I wouldn't soft-soap it either – I'd be as blunt as hell.

Since there is a history there, of course it's favouritism. Can't be that good grandparents if they're favouring one grandchild over the other. Pil should be bloody happy you are still inviting them away after open and blatant favouritism.