Invited to celebrate my twin sisters birthday

Assuming you have a group chat with all friends & your Sister in it, I'd message asking everyone 'What's everyone wearing for OUR birthday party?' or 'How's everyone getting to OUR birthday party?' something general but letting them know its a joint party for you both.

I’d not go, ‘Sorry, Tom is whisking me away to the Cotswolds/Paris/anywhere for the weekend to celebrate. We must set up something when we get back.’

That way your DSis will have to spend the whole night explaining to everyone where you are.

I can't tell from your posts if you've ever said out loud to anyone else (other than your sister) that it's your birthday party too. If you've not been correcting people when they've said it's your sister's birthday party then you need to step up and let people know. Stop being "awkward" - it's not an excuse.

Find some more intelligent friends, they're clearly not the brightest bunch.

Find new friends

That could come across like @Megssy was angling for a present ....

Is there a group WhatsApp for the event? If so I would just pop a message on their saying ‘I just to clarify that this trip to a JOINT birthday celebration!

Keep focusing on making new friends and keep quiet about it.
Keep your business to yourself.
Stop hosting and paying for everything, take a step back.
She has organised her birthday away, decide if you want to go or use the baby as excuse not to.
If you do go, plaster a smile on and just get on with it.
Organise something for yourself in your home on your terms.
But really focus on carving out a new set of friends, you will be much happier for it.

As a twin, I get it. If both twins are going to be at the party i would assume it would be a joint celebration. My twin and I don’t really celebrate/see each other on our birthday as much anymore now we are older, even though we’re very close; but if we were going to be together that day, it would be a joint event.

Your not being unreasonable but your not going to change your sister. If I were you I wouldn’t go. Enjoy your birthday with your family, and arrange a dinner party with your friends. I’d come up with an excuse about having just had baby. The emotions around your situation are complicated and entrenched. You don’t need to put yourself in an uncomfortable situation just to have a few drinks and a laugh. I wouldn’t even give your sister the satisfaction of knowing she has affected you. Good luck with however you proceed.

Maybe just have a little think what it is that you are actually worried about here.

That all the friends condone that you get ignored on your birthday?
That she steals your friends?
That she/they don’t actually like you?

Organise your own party, and enjoy both

And this needs to be nipped in the bud otherwise the weekend will be insufferable for you. Celebrating her, presents for her, cheers to her etc while you sit there and do what? Seethe? Cry? Resent her?

Sort it out now to avoid future issues.

If it was me I would decline the invite since she wants to celebrate her 40th birthday separately. Simply invite the people you choose to a event of your choosing.

This is so weird and you are right to be annoyed!!! I

Bonkers. What are they thinking

"Awkward" is your personal response rather than anything to do with her. Lots of people have explained about other twins who have separate parties, so it really isn't "weird". She's shared 39 birthday parties, have you checked if she is willing to share this one too? If not, can you really not manage to let her have one party that's just for her, while you have a different celebration?

Maybe its a secret surprise party for you

If you're so laid back, why aren't you willing to just enjoy your sister's party and set up a separate event of your own?

It sounds like as you've not been around as much that perhaps your sister has been talking about you behind your back and getting between you and the friend group.

The only thing you can do is speak to your sister.

This doesn't really align with your other comment:
She's a natural organiser. Lots of ideas generated in social gatherings and you can be sure my sister will follow instantaneously with organising and sorting everything. Not me being lazy or unwilling just that she is super keen. Whatever the occasion, new year, weekends away, family occasions you can be sure she will sort everything out and more. That applies to everyone not just me. I always offer to help behind scenes . Ie might do the food or something

I get that life isn't neat and tidy, but it can't be both you usually host everything AND your sister usually organises everything. So most likely you both like being "the hostess" and having attention, and that is the root of what's upsetting you here. This will be a tricky one to take on board - but how about considering the high probability that nobody else cares who is hosting? Why are you both so desperate for attention from others? Is it just the need to distinguish yourselves from each other, or something more?

Of course the opinions are mixed. We know very little about the situation and knew even less before your updates on this page, so everyone is responding mostly based on their life and their own situation.
Now it is up to you to sift through it and decide what and how is relevant for you.

What do you want to do now?
You can ask your sister privately, whether it means you should start planning your own birthday celebration.
You can come and observe the chaos.
You can can pick any other suggestion.