AIBU to not pay for them?

It's family. Way to make the Step GC feel less than.

So how many relatives can the OP’s son in law just expect her to foot the bill for? What if he wants to bring his parents 4 siblings and a niece? The fAmiLy after all which means OP - NOT a family member - should pay for them right?

Also why can’t their dad pay for them?

Sounds very tight and you need to rearranged advising why so it does not happen again.

Wrong, OP. If you are taking a family out then you pay for the whole family. Anything else would just be rude.

Agree with this. I wouldn't have expected my mum to pay anything for my step children when I was with my ex. If they had been part of the initial invite that is different. No one should be adding in extra guests when someone else is footing the bill without offering to pay.

This. It's perfectly reasonable to say that it's out of your budget. It also means that this won't reoccur without a discussion.

Split the bill - simplest
Rearrange but then you don't see your GD on her birthday
Move to a cheaper venue/activity

If however, this is more about not wanting to pay for them but being able to without much financial difficulty then pulling out will be obvious and rude.
Very rude though not to offer to cover the cost when advising you the weekend arrangements had changed assuming you'd told them it was your treat. So if there's form for taking the piss generally, I'd simply say things are tight.

This is the thing - as someone who was a step child several times over, I had plenty of my own grandparents and didn’t need more. Frankly we all have doe eyed views of children coming and “being part of the family” but so few actually care or want to. Some are still getting their heads around them being Family Number 1 when mummy or daddy has found Family Number 2, that very few are eager to get new grandparents they have no natural bond with

Yet another "they're forcing me to have feelings and obligations I don't have or want" blended family thread.

If money is tight, the people who should be paying for the step children are (1) their father (2) their mother, via pin money (3) the daughter (4) the MIL but then the DD and SIL pay for themselves.

If money isn't an object, OP would be cruel to make a point of not paying for these children.

Nothing will convince me that blended families are a good idea or successful for all concerned.

If I couldn't afford the extra people I would let the adults know that the plan is now pushing my finances and suggest a chippy Tea, KFC buckets, Maccies etc at home where you can have your own bottles of pop/tub of ice cream etc. It's more important to be a happy family group than to go somewhere more upmarket.

Why? Why can’t their father pay for them? OP did expect them coming and hasn’t approved paying for them

Not sure what age the older children are, but if 18 or under I think you should be reticent about asking their parent to pay for them unless you truly cannot afford it. At any rate do not make any such suggestion in front of the children concerned (or calculate the cost split openly in front of them when the bill comes). We have had this sort of thing happen occasionally and just sucked up the cost of the step grandchildren although we rarely see them and don't really have any sort of relationship with them.

The SIL teenagers (hopefully) have two other sets of blood grandparents, so they're hardly missing out in life by not knowing the OP!
All the people saying it's mean, need to read the OP again! It doesn't sound like she has a problem with them going, just that she hardly knows them and hasn't budgeted for two extra! And why should she?! Plans have changed, OP didn't change them?
Some people will literally jump on any perceived tiny detail without actually taking in anything from the whole OP at all!
They see the words "step" anything and to do with money and it sets them off into a narrative that doesn't even exist! 🙄

do you always spend other peoples money for them with out consideration?

just say

oh shall we rearrange the meal for another time then? as you'll understand I hadn't accounted for the extra meals and drinks

inviting extra people to a birthday celebration and expecting someone else to pay isn't courteous and unless the father has already indicated that they are paying tenth expectation would seem to be their they can add. exxtracosts onto you - which is unfair

Of course you pay for them.

Your DD is being really cheeky to have thrown this on you without offering to pay for them. Really, unless the husband’s Ex was having surgery or something they shouldn’t have agreed to have the step children just to accommodate her when they already had other plans, though that can be a tricky line to hold as goodwill makes co-parenting a thousand times easier for everyone.

You do not have to treat step grandchildren the same as biological grandchildren, especially if your SiL hasn’t developed his DC’s relationship with you. And you certainly aren’t bound to take on extra costs than you’d planned because someone else changed their plans.

I would just say to your DD that it is now outside your budget and suggest something else instead. E.g. rearranging the meal, giving your DGD a present or an individual day out instead, etc.

I don’t think you’re being rude, OP. You’re happy for them to be there/be included, but that wasn’t the original plan and if you can’t afford the additional cost, you can’t afford it. I’d definitely have the conversation ahead of time, though, so the SGC aren’t made to feel awkward on the day when the bill arrives…

Because why should she be expected to pay for two extra people she doesn’t have a relationship with?

She offered to pay for four people. Adding an extra two and expecting her to pay is taking the piss, whether she can afford it or not.

And in this week's episode of why "blended families" are a nightmare...

These two extra children are NOT the OP's grandchildren. They are her grandchild's half siblings, who have another granny somewhere. Entirely reasonable to either rearrange entirely or ask the dad to cough up.

I would suggest going somewhere cheaper so that you could afford to pay for everyone. If they offer to cover the extra meals, great, if not...go for a cheaper meal so that nobody feels left out.

The point is that while they’re family to each other, they don’t all have the same family members and won’t all have the same relationships.

Their younger sister’s grandmother doesn’t need to pretend that she considers them her grandchildren, any more than they need to pretend they consider her to be their grandmother, or their stepmother to be their mother.

Then it's absolutely fine to ask for financial assistance

But she phrased the OP as a reluctance to pay for them, as outsiders who she has 'no relationship with'. Rephrase it as 'would I be unreasonable to ask for help towards the cost of this family meal'

Why would the children not be going out with their half siblings maternal GPS? Is that not what this whole threads about? That the half siblings should absolutely NOT be allowed to have any family celebrations for their birthday with the maternal family that don't also include their step siblings? And that the maternal GPS should happily fund these social occasions?

I think it’s cheeky of the parents to bring the stepchild if they knew you were paying without stating they would pay for them beforehand. In this situation just offer to pay for all the children, and adults can pay for themselves.