To be thinking something so awful?

Only if you let it be. Christmas in my family is a week long affair. I'm sure poor miserable daddy could celebrate on boxing day if he actually wanted to but it seems he's very emotionally manipulative and doesn't want to.

His ex wife sounds quite unwell tbh.
Your sisters undoubtedly have trauma tied up in all of this (it's essentially coercive control).
Your dad isn't helping the situation. It's been 4 decades, it's time to get over it or do something different (e.g. Christmas eve)

You are not unreasonable to wish she would just vanish, that’s a normal thought. No reason to think you would act on it, however dreadful she is, especially as you know she is loved by those you love (sisters).
However OP, honestly you need to stop buying in to this drivel. Your dad is behaving appallingly and somehow convinced you that it is normal justified behaviour. It absolutely is not.
Almost every person I know has an imperfect Christmas Day, not seeing everyone we’d like to, due to divorce, blended families, death, geography, work, countless reasons. Most of us just have to make do and spread the meet ups over various days. That’s just life I am afraid!
Your father is incredibly selfish and self absorbed. Take a step back and make a decision to do better than him, or you will also spend 40 years spreading misery!

Only your sisters can decide whether they will stand up to their mother and her histrionics or not; you can't make them. I know it gets you down, but this is what happens sometimes when people divorce and remarry. You can't fix it; try to let it go.

Well, OP's mum has allowed her husband to make their daughter feel like second best, and been entirely complicit in giving her shitty Christmases her whole life. Surely most mums would be pretty fucking furious if their DH spent Christmas sulking in a corner, telling their young DD there was no point making any effort just for her. I really, really hope this isn't true as if it is, both parents appear horribly cruel. I can't really blame OP for wanting to kill off her dad's ex if she believes it's the only way she'll get an actual Christmas, though I suspect she's more likely to get one by leaving her parents to it and finding a new group of friends or family who actually value her presence.

Not many pps have addressed the actual AIBU of wishing the ex dead though. That's been largely ignored as irrelevant because of the more important attitudes and dynamics of everyone involved in the story (including the OP's character) that have perpetuated such an unhealthy acceptance of "horrendous" Christmasses for forty years without doing something about it.

I'd expect a qualified therapist to do the same tbh.

No one wants two consecutive Xmas days how ever much they love Christmas

That's categorically not true.

I can’t believe this thread is still going. Is this even real?
The only person who sounds remotely likeable here is OPs mum.

Ask your half sisters on boxing day without your dad being there ... next year can our Dad (and me) just have one family christmas ... just one!

I think doing Christmas on Christmas eve is a good suggestion if the issue is that they are tired and hungover.

But as a more general point. Having a good Christmas is a choice. If your dad wants to ruin Christmas day for you and your mum because he's sulking about something that will never change then that's not fair on you and you are within your rights to do something different.

Who else do you have in your life? Partner? Friends? Proactively create the Christmas you want then you can invite your parents and sisters! They might actually come if it looks fun!

Of course dad's can miss their children, no one is saying otherwise. People are saying that after 40 years he needs to build a bridge and get the fuck over it. He's ruining things for everyone because he enjoys it at this point.

After 40 years you can't change their mum, I don't agree with her actions but your dad and your side of the family are allowing the mum to control all of your lives for 40 years now. You can control your actions and plans, and don't tell the mum so she can't ruin it.

For example, this year I celebrated Christmas on the 21st Dec, I don't have my kids for Christmas. We had such a great day, and this morning they go tk their dad's. Doing our Christmas before the 25th means I'm not as grumpy and sad as I thought I would be, I got to enjoy Christmas without my ex trying to destroy it, and we as a larger family enjoy our time together. We did Christmas eve. Boxing day. The lot.

After 40 years it's time to make a change. So after this Christmas have a talk as family and work out a new plan. Lots of kids have two Christmasses now, and this prevents your sisters, BIL and nephew from being exhausted on Boxing day with you guys.

At the end of the day, the 25th Dec isn't working for you all as a family so just change the date and enjoy yourselves. Stop letting that woman control the whole situation, allowing her that level of power for 40 years is madness!

When do your poor bils get to see their families at Christmas.

There’s a touch of misandry in this thread. If the op’s dad was her mum and had never seen her children on Christmas Day for 40 years because of the highly manipulative actions of her ex husband, you would all be saying how awful it was, not telling her ‘mum’ to get a grip. Dads can also miss their children.

That said the time for your dad to fight it was 40 years ago and when your older sisters were children. The dye is set now and you can either continue to allow it to ruin Christmas or not. As it’s just the three of you why don’t you plan to go out? Or go away for it somewhere fancy even? It’s established the older sisters can’t come but you don’t all have to sit around being sad about it every year for 40 years!

So hang on a minute. Ex wife commands Christmas day or she will kill herself - your dad doesn't get Christmas day with all his kids as a consequence. She won't do Christmas day with all of you or she will kill herself - dad still doesn't get Christmas day. You guys get boxing day instead with your sister's but your dad isn't happy. You guys can't do Christmas on Christmas eve again because she would kill herself. It's been 40 years since ex and dad split. Your mum arrives 5 years later. You came along say 30 years ago. You say she blames you for splitting her and your dad up but refuses to interact with your dad since (which if she is still pining over him would be optimal make a move time). Your sisters refused to point out to her that her maths is wrong and that she is delusional that she and your dad would ever get back together whether you were here or not and they are fed up having to be emotionally blackmailed to not ever be allowed to see their dad on Christmas day? Honestly, she has no reason to change her behaviour - why? You have a sister problem, not an ex wife problem. Until they make a change, your dad isn't going to get the Christmas he wants. Why should she change that behaviour? You also have a dad problem - his wishing for a Christmas with all his family present (which isn't wrong by the way) is making you feel inferior. Have you ever sat down with your father and sisters and explained how you feel? That on Christmas you get to feel like an affair baby because ex wife blames you for split and sisters take her side so she isn't alone on Christmas day meanwhile your dad puts in less effort/wallows/makes it obvious he isn't happy FOR YOUR WHOLE LIFE so Christmas day is something to dread for you? If you haven't spoken up about this, do it now or forever have a rubbish Christmas. And if you have and nothing changes, why are you putting up with this behaviour? And where is your mum in all of this? Why hasn't she told him to give his head a wobble? It's been 40 years for heavens sake. What did he do for the 10 years before you came along? I feel for you, I do, but you are blaming the wrong person here.

Your dad is still a child, and that’s why you feel responsible for his feelings. You are not responsible for them.

Why doesn’t your Dad do the regular childcare for his DD, then their mum can’t hang it over the DD’s head as a threat to remove if they don’t go to hers on Xmas day?

Why should he get the fun days if he’s not putting in the (presumably free) labour that comes with being the default childcare for your GC?

Have Xmas day at your Dad’s on Xmas eve.

His Dds could stay sober on Xmas day (or at least drink less) so they’re not hungover on Boxing Day?

There’s so many solutions to this “problem”, but it sounds like you all just want a big moan.

This is a melodramatic pity party.

OP after my parents were divorced and my Dad remarried we always spent Christmas Day with my mother who would otherwise have been alone.

We spent Christmas Eve at my Dad's which was always an enjoyable occasion.

My Dad never showed any resentment about this and just got on with it.

Your Dad sounds a bit self absorbed

Your poor mum.

Just move the whole thing to Boxing Day. We often did when I was growing up as my mum often worked on Christmas Day. A family member does Boxing Day now as it guarantees she gets all her adult children & grandchildren. They all see their in laws on Xmas day or do their own thing - it’s become a family tradition. I did a few times when kids were younger to meet needs of my eldest. I’n thinking I prob will next year as it’s easier with my eldest and as my kids get older will work well around them and their partners. It can work well if you just embrace it.

So your Dad spoils your Christmas because he cannot have all his family around.

Did he do this when you were a child OP, after he left his daughters and his first wife?

I feel sorry for your sisters. They have two toxic parents that are trying to manipulate them into spending Christmas with them, if I was them I’d stay home and not see either.

Why doesn't your dad stop feeding the misery and do what he wants? @iamsoashamedofmyself

And his ex-wife wants the same - all of her children under the one roof - so why should his wishes be more important?
He had a family with his first wife, left them (decades ago), had another family, but still wants to dictate how the first wife gets to spend Xmas Day - ie without her children.
He sounds awful, as many people have said, acting like he’s miserable and sorely-wronged…and you’re making him worse by indulging his childish petulance.
As many have said, you could have the day together on a different day, and CHOOSE to enjoy that day regardless if it’s a day later.