What approach to take?

No way would I put myself through that.
I would stay in my own home and be relaxed instead of walking on eggshells for 4 days.

Could you perhaps stay in a hotel so you can escape for some of the time?

4 days with non speaking person , nope , not even for the GC , they will be aware of the atmosphere

Well I would say don’t resurect anything about the past.
Avoid all contentious topics, ask if any help needed, etc.
Follow their lead, try to stay in a calm friendly mode. Best wishes, it’s never easy when bonding with anyone who is on a different wave length, but it is possible , I use to remind myself of how I felt at times around my MIL .
That old saying has some truth in it :- if you have a daughter you have a daughter for life, if you have son it’s until he has a wife?

IIRC you are the family visiting from France?

Are you staying with the family or could you get some alternative accommodation just for sleeping in at night? To give you and DIL a little breathing room?

Does your son know what the issue is but is refusing to discuss it? I doubt he and DIL have not discussed the issue. How do you know you were invited reluctantly?

Never mind having to walk on eggshells, I would walk on burning coal for my Dgc.
I would go for the four days, be as helpful as you can without being intrusive and I would treat Dil normally i.e. as if nothing was wrong.
So, I would arrive, after greeting excited Dgc, I would give Ddil a big hug, lovely to see you, thanks for having us, you/the house looks lovely.
Then I would carry on the whole time as if I hadn't noticed she wasn't speaking and include her a lot, for instance when talking to Dgc say something like, I wonder if mummy would like that or let's see what mummy says.
You get me? Keep mentioning her, while talking to the Gc, basically be extremely kind to her.
I know that you probably want to strangle her but really try to build bridges. She's obviously hurt by some real or imagined slight from either you or your Dh, so be the bigger person.
Good Luck!

They have invited you, just go for goodness’ sake.
If you decline or make a fuss about staying somewhere else, your DiL and DS will be quite justified in being upset!

Crossed posts, buffy. I agree with your approach.

Well evianers, at least you have been invited over. Your son obviously thinks a lot of you and wants you there for Christmas.
I have a disastrous relationship with one of my d.i.l and I have no support from my son. He says the nastiest things to me and I feel we are reaching the end of the line. I have just had a nasty text from her accusing me of not caring for the grandchildren just because I didn't ask how the little one was doing at her new school. I was told to keep away and so I didn't ask. Now it's been thrown back at me.
Anyway, I feel I want to move far away from everyone. All of us live in the same town and I just hate it. Lord, take me away from all this, please !! He never listens anyway?

Some good advice there, Buffybee. Disarm her with kindness. After all, you catch more flies with honey than with vinegar.

I would email or text her to say you are delighted by the iinvitation, and want to fit with her plan, so....
when would be a good time of day to arrive?
to leave?
and, that you gathered from DS that the idea was to stay with them but wanted to doublecheck? if not, can she recommend a good local hotel.?

Make it friendly, light, and personal. Maybe ask what GCs might like for Christmas?
Then avoid anything contentious, and don't let alcohol take control of your mouth at any point (I have no idea whether you drink at all, or at what level, but alcohol is a massive factor in seasonal family disputes!)

Yes, it is playing it "her way" but the invitation is an olive branch. I would accept it as such, and try to move forward with a fresh start.

Though in your case, crazyH, it seems you can’t do right for doing wrong!

I don't see how it can be made worse really. Would you still see your Grandchidlren through your son?

Have you asked your son what's wrong? Does he know?

Thank you for the interesting answers so far : yes, we think our DS knows what is wrong but he is torn between his wife and his parents here trying to keep the peace.
We fly from Geneva to Luton, so must arrive to coincide. Reluctance is from DIL's, not DS's side. We have given a substantial sum of money for improvements to their house, so do not feel as though we want to spend the festive season with someone unknown [Airbnb, B+B etc.]

Yes Maggie, spot on.

I think ChaosIncorporated has made an excellent suggestion and personally I'd go with that.
Your last sentence is a bit odd, so what if you contributed massively to their home improvements, you'll be able to see them and admire them over the course of the 4 days. You don't need to live in to do that. I thought the idea was to build bridges not inspect where your money has gone.
Offer to stay elsewhere it will certainly make things go more smoothly.

The killing with kindness strategies suggested by buffy make sense and have certainly worked for me in the past in a number of scenarios!

I agree with chaos try and keep things light, you may be able to find the cause of the problem once channels are open. Your gift of money for their house was just that - a gift.
Attaching strings is not going to help. I hope things improve for you all.

I agree Maggie.
It shouldn’t be a battle but behaving beautifully always wins.
Give her no weapons.

If you don't know what IT is, how will you be able to know If you are doing IT again?
I would need the air clearing comprehensively before I visited.
If you are flying over, you will not be able to just get in the car and drive home if things blow up.

Stay in a hotel. It obviously makes your daughter in law unhappy and anxious if you stay with them, so make it easier. You can all breathe a sigh of relief as to go back to the hotel each evening, and you will see your grandchildren during the day. I don’t see what the money for improvements has to do with it.

I agree with annsixty it could be awful if it were to happen again. Who extended the invitation? Did it just come from your son? If your d.i.l. has made her reluctance obvious I would need to get to the bottom of why she'd not spoken to me for 9 months before accepting.

Walking on egg shells for the duration of your visit would be extremely stressful IMO and walking into a possible mine field, even worse.

Gosh. If I had invited my MiL to stay and her response was ‘no thanks, I’d prefer to stay in a hotel’ I would have been really hurt.
Do you really want to make things worse?
How hard can it be to just act pleasantly for four days?

Easy to act pleasantly , IF you know what triggers to avoid , Your son needs to clear the air