Party etiquette - will this cause a family argument?

You already seem to know the answer. So, I think I would invite them all but with a detailed explanation of what the event entails. Then you can go on to say, if anyone doesn’t want to attend the activity then they are very welcome to join for the meal/cake afterwards - if that’s what you’re planning, of course. Wouldn’t that be a good compromise?

How is some of the children not liking the activity 'family politics'?

All or none OP otherwise you will probably cause upset.

Well @GiveDogBone given youre vitriol here, I can imagine youd probably have that response any time she wanted to something for herself..
'How selfish of you!!! #bekind others before you always!!'

Are you seriously suggesting she won’t have any other opportunity to do what she wants at any other time of the year. What a stupid comment.

I mean if her best friend was deaf, should she go to a musical for her birthday?

When? If she has to put other people's wants above hers on her birthday, what day does she get to do what she wants?

This is a teaching moment for your daughter. Her birthday party is not just about her, it’s kind to choose an activity that the other children will enjoy as well. She can do the other thing at another time.

Why cant you invite everyone but tell your sibling no hard feelings if their children wouldnt like it

Invite them all ...

I would personally invite them all and then leave it up to the parents to decide if their children want to go or not.

Ring them up and ask if they'd like to be included? Just come for tea? Meet at another time?

Invite everybody but explain to your sibling what the activity is and ask them to check with their DC that they will be ok doing it but if not they could join you for the cake part afterwards.

Saving £60 - not worth the offence. It’s not nice to exclude. Surely they can decline if it’s not something they’d like?
I have a child with autism so he can be sensitive and I’ll tell you now it hurts like hell when he has not been invited to parties purely because others assume they know best about what he will/wont enjoy.

1. he’d like the invite and to know he was included.
2. we’d politely decline if its something we know he wouldn’t enjoy it.
3. sometimes he would enjoy some activities you’d be surprised at. Or could enjoy them with adaptations (ear defenders for example).

I’ve had other parents tell me they didn’t invite him to an activity party because their child said he wouldn’t enjoy it (we’d been 2 weeks before and he loved it). I didn’t say anything but I won’t forget - I doubt your sister would.

I would definitely spend £40 on avoiding family conflict

Ah sorry @Sprogonthetyne just saw that full sib can't come unless your dm comes too, which would then poss cause issue if she see half sib and kids?
Surely she realises that half sib won't have had any saying in her being born?

I can’t believe not inviting the full sibling’s children (or at least having a conversation first) is even entering your head! Of course your sibling would be hurt!
Just communicate for goodness sake. Either that, or don’t invite any of the cousins and invite school friends instead.
I’d love to know what this amazing party is btw!

Assuming something like LaserTag run around for a bit, and then food. Is it possible they could come just to the food part?
I think 60 quid, is not too steep a price for avoiding a family conflict to be honest.

Just invite them all, explain to your sibling what the party will be like and they don’t need to come if they won’t enjoy it.

2 out of 3 children may sit out which is £40 but it’s £40 well spent if it avoids family drama.

It’s all about communication. Phone full sibling and have a conversation about it. They could actually check with the cousins about whether or not they want to do the activity. If they don’t you just make plans to see them another time.

I would say “We are doing this and obviously Jemima and Jeremy are invited. If you think it’s not going to be their thing though, I know DD would love to see them for birthday cake/tea/picnic/park trip (whatever you think would be jointly enjoyed) on this date? Let me know what you think they’d like best?

YABVU to not just invite them all!

It’s your dd party and she should do what she enjoys, the other dc and adults can come or not come but don’t make a drama out of it.

I really do think you have to invite them all. Or neither of them.

I don't think you can base stuff of children's personalities or likes and dislikes. Especially as they change like the wind. DS just came back from the party where I'm v good friends with the mum. It was a princess one and not his scene and I don't think he was that into it. However I'd have been sad if he hadn't been included!

20 quid per child seems quite steep so I think you'd be fine not having any cousins. I'd not expect my kids to be invited to every cousin party just the big free for all church hall ones.

Couldn't you just ask your sister before you include them in the numbers?

"Hey sister, Lily wants a football party this year. Would your guys enjoy it or should we meet up separately? No pressure either way!"

Invite them all and give them the choice whether to attend or not, this way there's no potential for a fall out.

Does your full sibling have a relationship with the half sibling?