AIBU to not pay for them?

How would it be fair to my GDC for the SC to get presents from both their sets of grandparents and from me, when my DGC just get presents from me and their one other set of grandparents?!

Because according to all the people living in these situations trying to justify how well it works, your DGC should be getting gifts from their half siblings' grandparents too, because it's only fair.

@JandamiHash That's my logic with DD. If I just take her, we can go somewhere nice; if I take the whole family, we’d have to be more price-conscious. A family meal and cake at home happens as well, of course.

Nowadays she likes a facial or a mani/pedi anyway and I can’t see DS being interested.

People always seem to demand grandparents suddenly begin treating strangers like family, whilst forgetting that these kids already have two parents and (usually) two sets of grandparents!
If my DD married somebody with kids of course I'd be kind to them and pay a polite interest in their lives but I certainly wouldn't be a grandparent to them why on earth should I?! They will have their own sets of GDPs. If DD went on to have kids of her own with her DH then obviously I would buy the SC a token gift at Christmas so they're not left out but again, it wouldn't match the sentiment & generosity of my present/s for my GDC because.....they're not my GDC! How would it be fair to my GDC for the SC to get presents from both their sets of grandparents and from me, when my DGC just get presents from me and their one other set of grandparents?!

Well exactly. And when the older two are treated by their own grandparents is the smaller half sibling expected to be included in all of that too? What about Alexa and Bill, who have a child together, Alexa already having children with Colin and David, and Bill has kids with Eva and Fiona?

When adults choose to create these situations by having multiple "blended families" with half siblings and step siblings and children with multiple grandparents you cannot expect everyone to be delighted about suddenly having to accommodate extra children for occasions like birthdays or buy gifts for Christmas, especially when money is tight. And although you might be kidding yourself on that your blended family is happy and equal and everyone is just the same, you cannot demand that your extended family plays along.

Or he could just pay for his older children.

Good God how the other half live eh?

OP it is very rude of your son in law to invite extra people along, who the hell does that when someone else is paying ? Height of bad manners.
I'd tell your DD that your budget won't stretch to that so they either rearrange or pay for the extra kids.

I feel sorry for the older children and for you, you have all been put in a situation not of your making, I think you paying for children and adults paying for themselves is a fair compromise

Treat them what way? She’s happy for them to attend, but she shouldn’t have the suck up the costs of a couple of last minute guests.

They’re not her grandchildren, and they barely have a relationship with her. Given that their father was happy to arrange to go out to celebrate without them, this doesn’t seem to be a problem.

Good point. As for people saying “Rearrange!” - presumably the date is picked because it’s the GD’s birthday.

I doubt OP will come back to clarify and I don’t blame her. Loads of plonkers boo-hooing on behalf on kids they don’t know and behaving like stepchildren have something missing from their lives that other random people are obliged to provide for them

@Vaxtable Going forward, I’d advise the OP to treat her granddaughter on her birthday and not her parents.

I treat my DD (19) to various things for her birthday and sometimes say that she can invite a friend.

Lots of people saying 2, OP doesn't specify how many. Could be 2 or 4 or 17

The other thing that stands out from all this, which nobody else has even mentioned, is that no-one has actually thought about the birthday girl in all this! Why not change venues, dates, have pizza/burgers at home, Maccies or KFC?!
What if she's actually chosen the place she'd like to eat at, for her birthday and is now having it totally disrupted just to accommodate 2 people that weren't even going in the first place! Step siblings or not!
Another case of people not taking things into account when jumping in and putting forward stupid arguments!
Too quick to jump in and call the OP mean and not inclusive!

Surely they won't cost much, they're not drinking age are they?

Means nothing these days. In these parts a pint of lager is only about £1 more than a pint of coke

Treat them like what?

Ultimately “step grandparent” is a meaningless term much like “step cousin”, bestowed upon people who have had no say whatsoever in the choices of the relative who started a relationship with someone who already had children.

Yeah but that’s not the case with the OP is it

I think there isn't really a way to phrase it that doesn't sound a bit off.
Though if they're aware you're paying it's not great manners to add extra people. But equally the other kids couldn't be expected to stay home alone while you lot all went out.
I'd either rearrange for another day, or accept the fact you may have to pay for the extra two kids. Surely they won't cost much, they're not drinking age are they?
You'd hope that on the night the dad would produce his wallet and offer to go halves or something. That's how my family would do it. But if you genuinely can't afford the extra two then rebook it.

Wow. My step grannie was the only one I ever knew. She treated me as her own

Lol. How are you so confused?

Ask to go 50:50 with DD and her DH as they are bringing extra and you can’t afford all of them.

Treat them like what? Shes happy for them to come.

How would you like it if someone just added a huge expense you could really afford to a treat you’d offered to give them?

Why would kids even know or notice who is paying?

I wouldn't be paying for the extra children, no. They're not your grandchildren. They presumably have their own 2 sets of grandparents.

This is so horrible. Why would you treat your step grandchildren like that OP? Would you want someone to treat your grandchildren in that way? Vile.

And on another threads having children in restaurants at all is akin to bringing a mass murderer to Sunday school.

If they desperately wanted a bond with their step/half (we don’t know) sister’s other grandma then I’m sure OP would have picked up on it