Trying not to blow up at my mother

You acknowledge that your sister is irrational and you don’t like her, yet you are considering having a party for your mum, to one up your disliked sister?

ask yourself why you are making such decisions. What is the purpose of this celebration? Your mum? Or to stick the finger up at your sister?

I would hope that the intention is to celebrate your mother and if that’s by having a trip, then do that. You then need to manage your feelings of jealousy and exclusion. If your relations are celebrating in a different country, then that’s just a product of your distance, maybe. That should be OK.

Your mum doesn’t seem too bothered by parties - she has chosen a trip over a party with you.
Maybe to her it’s simply no big deal? You live in a different country, so you have chosen to distance yourself geographically from your family. It’s completely normal that they get on with life while you’re not there. What seems to you as a BIG celebration was perhaps to them a thrown together gathering of those who live locally? Lovely that so many could make it, nice photos etc, but nothing to do with the children who live abroad.

Honestly, you’re giving this too much thought. You are celebrating with your mum the way you and she have chosen to. Let the rest of your family in their country of residence do what they like. It’s nothing to do with you.

Organise your trip and go on that - enjoy it and let them organise a party if they see fit.

That way, you're still getting to celebrate with your mum.

With all due respect, and please don't take this as a dig cos it genuinely isn't (and your sister sounds like a dickhead), they also have no duty to arrange around you and nice sister as it's your mums birthday. You have a trip planned, and can either make the party, or you can't, it's surplus as you're already celebrating.

Get a picture on your trip, fuck their party and leave them to it.

You will never get a date that suits everyone. Tbh if a family member has a big birthday coming up I generally ask if there are any plans, any dates I need to keep clear and so on. Especially if there is travel involved.

This for me is a key factor, along with the fact the two of you live abroad. I would organise a party, frame it as a family party for everyone to get together as the wider family never all get together. Then have a cake for your Mum you can bring out.

You know your Sister will do a party and exclude you so preempt her.

Your DM is emotionally immature?
You are in your mid 40s witting about nice sister/horrible sister.
You don't get that someone might say they don't want a fuss or a party, they might mean it, but if a surprise party is organised, they might actually enjoy it, or at the very least go along with it with good grace?
You've decided to move overseas, but still expect every family gathering to revolve around you?
I think it's probably you that needs to grow up

This is ridiculous.

Contact the wider family, say you are organising a slightly belated birthday party for the whole extended family on X date as you were disappointed to not be invited last time.

Ask the Aunts if they could help by arranging xyz.

Take mother for trip as well.

Because she doesn't want the row?

But you all live in the same country?

Just make sure you have lovely photos from the trip

It doesn't have to be her exact birthday but most people celebrate with a big party on the weekend either before or after their birthday. Surely if that doesn't work because you are in a different country you do something special when you can get together. I really wouldn't be expecting all the family to wait for a time suitable for 2 members. I would however expect them to be invited so they can make the choice of whether that date works for them.

but if your mums birthday, therefore party is in school time, you wouldn't be able to go anyway.

What does your mum say to you about it all?
I would be letting the Aunties know that you knew nothing about the party and didnt get an invite.

You've asked, and you mom's agreed to a trip. What happens if you find out there is a party being organised?

Do you cancel the trip that she actually wants, to attend a party she says she doesn't want?

There's no blowing up necessary here. You've decided to accept her for who she is so you can have a relationship with her. You cannot, however, make your horrible sister or lovely aunt plan a party in advance and give you notice - those are outside your control.

FOMO is something you have to manage when you live away from the rest of your family as well.

Well I wouldn't be happy if one child even an adult one hadn't been invited but other than saying that isn't very nice to exclude your sister X what would you want your
Mum to do. Walk out of the party. Say sorry rest of extended family who have all made arrangements to come to my celebration but I am not going?
And tbh if the last party for your Mum was a big birthday then presumably you have been harbouring these feelings for 9 years. Or maybe 4 if you celebrate 65. Just let it go. You can't control your sisters actions but you can control your response to them. See your Mum and extended family at different times

You’re not answering the question though. “You’d assume he didn’t have leave/money to come to both events”. But what if he wasn’t even given the option to attend the family gathering? Thats rude, right?

You want to be part of a big family party so organise one.

I think most normal people would be upset at that. And in most families it just wouldn’t happen!