To be hurt we are always the Boxing Day/Easter Monday house

No, because of the DIL’s disdain for me. I’d rather support the proverbial cat’s home.

If they want posh schools they can fund them.

To be honest, there’s 7 separate households here:

You/your husband
your ex
his ex
son’s family
Son’s in laws
daughter’s family
Daughter’s in laws

It’s probably hard for your respective children to coordinate if the above group of people don’t really socialise all together, they’re always going to be letting someone down on Christmas Day. Plus as they are the ones responsible for the grandchildren, everyone wants to see them. maybe in the future, they might be the glue that brings everyone together and host Christmas themselves? But for now, it’s probably exhausting for them too.

Haha yes this seems like sensible adult behaviour….wouldnt burn any bridges and then OP wouldn’t see any family 😂

Sounds fine they make the effort to come with kids every year. Does it really matter? It’s just a family tradition now and they are traveling etc.

Why?? Because they see them on Boxing Day not Christmas Day when they live an hour a way? Petty and pathetic!

This answers all of your questions.
The set up sounds perfect for the wants and needs of all involved.
Do you really want people to come to you on Christmas Day out of some sense of duty or ‘fairness’? They would be secretly resentful at missing the day they enjoy.

Maybe try having your son and your dh's daughter on different days? Your son on xmas eve and your 'step daughter' on boxing day. I know you say the cousins like to see each other but to your son they are not really cousins are they? He might enjoy some time with you without also having to socialise with your partners kid and grandkids. Also as others have said, your dh should fund school fees not you. I would stop doing this and then you won't be so resentful.

I would remove my share of this support.

Honestly you’re lucky you get Boxing Day, we refuse to travel so our family only see us in between xmas and new year, you said yourself the moving around is all quite miserable for kids.

I actually think boxing day is a lovely time for you all to get together. It is a tradition just for your family unit & so far everyone seems happy to come over on that day. You get a lovely day with your husband & then all the grandkids come over the next day. It's making the festivities last longer for everyone which is so lovely. I would love a big family christmas/boxing day. What a privilege to have a house full of healthy happy family over the festive period. Try to enjoy it & see it as 'your' family day.

Honestly, it sounds as though for Christmas Day for both families, it would be a massive inconvenience to slog out to you (at least a 2 hr round trip, presumably in good traffic), kids and presents in tow, on the day itself. When your son was younger, would you honestly have wanted to pack up the car on Christmas morning, drive an hour for lunch and a few hours with one set of grandparents, then get back in the car (with all the presents, unwrapped this time) and drive an hour home again so you could see the other grandparents the next day (who live conveniently close to their own homes)?

What if they actually want a Christmas at home? What if they don't like your idea of Christmas (which appears to involve going out to a restaurant - very expensive and stressful with young children, who will just want to eat as little as possible before going back to their toys). Would you go to them and do all the travelling? Would be worth asking your DS at least.

If your DS is heavily involved in his church, asking to pull him away for said church at the most religious holy days of the year seems like you are setting yourself up to fail. I would see about visiting them instead, and perhaps alternating with seeing your SD's family. If you really want to be with them on certain days, you need to have less rigid ideas of how that will happen. I would recommend getting the cousins together at different times of the year, i.e. start a tradition of an annual BBQ or similar.

Why can’t you go visit them on Christmas Day OP? Ask if you can spend the morning with one family and the other in the afternoon if it’s that important?

Focus on your son. Leave DH to deal with DSD. If Christmas is already set this year, invite him in September next year, before plans are made. If he says no, tell him that it’s only fair that families alternate, and that your feelings are hurt. Same for Easter—ask early this year.

Each family get £1000 a month from grandparents to fund their choice of living - and can't be assed to spend Christmas or Easter every second year with the grandparents only the left over Monday/Boxing Day.

The ship might have sailed but it is amazingly entitled of the parents.

1. You've caused Easter Monday with your DS by refusing to see him on a different day than DSD. Change that if you want to.
2. It is completely reasonable and unsurprising that your Christian DS wants to spend the most important Christian holidays with his Christian family. I love Christmas and Easter, but I'm not religious so of course they could be celebrated on different days! They are essentially winter and spring festivals for me.
3. Your DSD's in-law Christmas sounds like it is much more suitable for their family at the moment. This is why we generally go to my DPs rather than my in-laws. I want the DC to have a good time, primarily.

It sounds like you’ve got a solid option to do something that’s not Boxing Day or Easter Monday with your son.

Youre just choosing not to do that because your husbands daughter isn’t keen. You describe the various grandchildren as cousins. That might be your view, but it quite likely isn’t everyone’s. As I understand it the only connection between them is that their respective actual grandparents married later in life. So what you’re trying to pull together is two fairly disconnected groups of people. They’re all your grandchildren but they’re not actually cousins and it’s not unreasonable for some to see things that way.

Driving over an hour each way on Christmas Day with small children is madness if there’s a good alternative.

I think the advice along these lines is good .I actually think it's lovely that you have a blended family of sorts but they still want to spend time with you as adults even if it isn't your preferred calendar day.
However still worth exploring why it makes you feel second best.
Could you do something else special with your son's grandchildren over the holidays before Christmas? Eg start a tradition to visit Santa or similar?
Or in general start some traditions for yourself and your OH.

I’d second popping over for coffee in the morning before they leave or bringing a takeaway a few days before Christmas. It’s a horrible, emotionally loaded holiday for some and the pressure to be a certain thing, act a certain way, provide certain things can be dire. Personally - I’d rather hunker down with some sandwiches and the TV until it’s all over than fuss about who is where with whom. I think many of us have difficult triggering memories of this time of year which make it fraught with a sense of rejection and disappointment.

The media doesn’t help.

You will never be that important to your adult step daughter no matter how nice and generous you are, so all you can do is support her dad to see her and the gc whenever it works for them both. I wouldn’t be surprised should he ever end up alone, he’d be invited to join them every year. Can’t you each go off with your own families? Blended families can be so tough for everyone - such an artificial set up.

It’s only a day a couple of times a year.

I agree with this. Don't miss out on seeing your son when you'd like to see him.

How much are the other set of grandparents giving towards their grandchildren?

I have put YABU because people should not be so precious about Christmas. We are grandparents. When our children were at home we did Christmas dinner here. Now my children have their own children, they want to do it at their own homes. In fact they do it alternately and we get invited. One child is close by and one lives in London. If we go to London we stay in a hotel to give everyone space. We have told our children many times that if they want to have Christmas without us then that is fine but to give us notice. We have no objection to spending Christmas on a beach somewhere.

My MIL was actually a step MIL and my children called her by her first name but they still loved her. She was not insensitive enough to be ask to be called grandmother. She knew my husband had a mother who would have loved to have been a grandmother but didn’t live that long. If your step DIL’s parents live close enough of course they are going to go there. Who doesn’t want to not drive on Christmas Day. Also perhaps going to your house would be a reminder to your step DIL that she is missing her mother.

I personally think that's very unfair. I'd love to spend Christmas with my family every year, I do not enjoy Christmas with my in-laws at all, but I suck it up every second year because it's fair.

Instead if trying to change things up for them, which they would resent and no doubt find impossible, I would try to let go of your own resentment.
They have young children to consider. It’s hard for your offspring to fit you all in and it seems everybody has found a pattern that sort of works. It’s not ideal for you as you have Christmas Day without extra family, but you do at least see them the following day. And you have each other.
I would think it’s not entirely ideal for each of them either and they also wish it could be perfect but they go along with it. Having lost her mum, I’m sure Christmas is a difficult enough time for your DH’s DD.
I wouldn’t rock the boat about which day you see them at all. Just try to enjoy what you can.

But, as a separate issue, I would certainly have a think about your finances and whether you can afford to continue to subsidise school fees. Does your DH’s DD actually know that you contribute or does she think it all comes from her dad?

To be honest I would just accept it and pack those feelings of hurt away. We are the post Christmas part of our complicated family and it took me ages to realise that there isn't any other solution Christmas is such a tiny part of the year proportionally and worrying about it was definitely impacting on me. I know that all sounds like rationalising away something that's important to you but I realised that it was too stressful to dwell on it. Enjoy the time you have with them.

Travelling an hour on Christmas day with a 3 and 5 year old is a pain. Her parents are nearby and it means she sees her stepbrother and his kids every year rather than alternate years. It does make more sense logistically.

I would rather have whole family every year on boxing day/Easter Monday than have the main day every other yr.

The granny thing is a bit hurtful but also shows she is still grieving so fine. I think when you lose your mum young all the milestones she misses can be quite upsetting (that has been my friends experience anyway).

Your feelings are valid but notice then let it go, appreciate all you have rather than "getting the main day" (especially as the other 9 people involved are happy with current arrangements). Try not to waste any more energy on this.

I’m afraid I wouldn’t rock the boat. Traditions like this when changed can end up not happening at all unfortunately.

You know the reasons why so I wouldn’t fight it. The distances unfortunately are key. I would love to spend the day at my dads but locally we can fit in about 4 different family households in that time which would be wiped out to do his so we do Boxing Day but do love that tradition.

Why don’t you add in some new traditions with the kids? Perhaps a late November light show or perhaps an after Christmas panto.

i wouldn’t fight a losing battle against what works but build on it