To bring DD friend away instead of DSD

Not unreasonable. Her dad and her younger sibling aren't going. This isn't a family holiday, it's you, your daughter and her friend.

Then her mum needs to be a good mum and explain that she's had trips and hasn't invited her half siblings and that she's being silly and have a laugh about her complaining with her. Not encouraging it. Maybe dsd might feel silly if she had it pointed out to her like that if she's a nice girl in general, which op hasn't said she isnt

The 15 year old sounds spoiled. If she goes away with her mother on foreign trips and knows her other sibling (your son) isn’t going as it’s a birthday treat asking to go is super unreasonable. At 15 she should definitely understand why she isn’t invited. I’m hoping it’s a misunderstanding and she just doesn’t have the full picture.

I’d clarify with DaD’s mother to make sure no one is confused but hold firm on not taking her.

I agree it's a problem that needs to be sorted. Someone needs to explain to the 15yo that it is not unfair for her half sibling to have trips with her mother to which she is not invited, any more than it is unfair for the 15yo to go on trips with her own mum without her half siblings. This should be pretty straightforward for her to get her head around.

I was under the impression the mother had the problem with it not the father

Yes her mum could organise that. Nothing stopping her.
OP has organised and paid for her a trip for her DD, the ex could do the same.

I don’t even see why DH has to make it up to her, he isn’t going, it’s a separate thing.

Everything OP does with her children does not have to be replicated with SC, she’s allowed to have her own relationship with her own children. Excluding her from a family holiday is entirely different.

I grew up in a large family, we went on different trips and holidays. It’s normal.

But the 15 year old has been taken abroad many times by her own mum and the OP's dd wasn't invited along!

They seem to want it all one way only.

A mother is taking her child on a holiday, the 15yo can discuss this with her mother and father it is not up to the op to do anything she has not done anything wrong

But it's not a son.

It's a daughter.

That makes all the difference.

Boys trip v one man on his own with two teenage girls (DSD plus another female child and a man on his own). Understandably he would be uncomfortable with that and many parents wouldn't allow their 15/16 year old daughter to go abroad with a man. Not to mention sleeping arrangements etc.

She gets family holidays with both parents.

OPs dd never has until now.

The 15 year old feels like she has been left out treated unfairly. That is a problem that needs to be solved - how is what needs to be determined. Another trip/ a discussion/ something else etc

Except there IS no problem to solve

Maybe an opportunity to get to know her friends.
arent you a bit sad that you/ her dad don’t know her friends. Do you not have them over at your house at all? Maybe start small and have a trip out with her and a couple of friends.

Yes, I agree that they should balance it out in their own family. So, both girls will have been on trips with their mum. The younger ds should also get a trip with his mum at some point in the future. That would be fair.

If dad decides to do any trips with any of his children, then he should ensure that he does the same for each child.

Your DDs birthday treat so her choice of who goes with you both.
Two 11 year olds on a birthday treat us very different to and 11 year old and a15 year old. I’d have thought a 15 year old would understand that.

So many double standards where the mum is the step mum versus posts where it is the dad who is the step parent.

No, they should balance out trips in their family. What the other parents do is up to them and does not need to be covered.
balancing it out could be the son getting trips with each of his parents and doesn’t necessarily need to be straight away but in upcoming years.

It doesn’t involve any of her parents, though.

This Is a mother / daughter trip, not funded by DSD’s dad. If DSD wants her own trip, her mother can arrange one.

It would be different if her dad was involved at all. He isn’t.

The step-daughter's mother may not be able to afford a trip like that. This makes it unfair in my eyes.

It is not a stepmothers problem to make up for the DSDs lower earnings.

So if the dad has to balance out any trips or treats offered to the 11yo by her mother, by offering comparable trips or treats for the 15yo, does he also have to balance out any trips or treats offered to the 15yo by her mother, by offering comparable trips or treats to the 11yo?

As it could seem that they are being treated unequally otherwise, with one child getting twice as many trips and treats?

It is a trip being arranged as part of her dads family and not her mums. In order to keep it fair each child in the family should get a trip with a parent.