To bring DD friend away instead of DSD

Who the fuck does the ex think she is, asking you take DSD?
This is a treat for your DD's birthday, not a whole family holiday. I'd tell her to take a hike and make clear she's acting like a spoilt ex whilst setting a bad example to her own daughter.
Sadly I suspect ex still thinks she can call the shots and it's a power issue and done to make you feel guilty thus spoil your trip.

I hope you have a wonderful time with your DD and her friend and don't give it a second thought.

Has DSD ever been away, with either her mum (and family) or her dad (and family)?

I think YANBU. But if DSD has never been away then I can see why she would be hurt. Watching your sibling go on holiday must be hard.

But I assume DSD has had family holidays?

Invite your DD and her friend.
It is her birthday and she is inviting one friend.
Fifteen year olds would normally not really hang out with two 11 year olds and call that fun, anyway.
It is not a family holiday.

Do something nice, or your HD can, for DSD on her birthday - inviting her and one of her friends to a weekend that DSD would really enjoy..

I’m surprised no one has sparked on the fact a 15 year old wants to apparently voluntarily spend time with an 11 year old!! I cannot imagine a 15 year old wants to actually spend time with an 11 year old and her step mother (no offence, OP).

DSD really wants the equivalent spent on her. She doesn’t actually want to go to Rome, IMO. She’s jealous of the trip and costs involved, not on missing out. That, is for her parents to fix. Not for you to fix

I would tell her that a) it’s DD’s birthday present from YOU and b) DD’s friend is contributing to expenses and c) She is welcome to take her own daughter whenever she wants to go to Rome.

You're absolutely not being unreasonable! It's your daughter's birthday treat, not a family weekend away. It's not like your youngest is going too!

Sorry your husband not your partner.

The ex is a cheeky fucker. Don’t second guess yourself OP. This is a birthday treat for your child. It’s absolutely fine to go as you have planned.
Does the ex behave unreasonably in general or is this is one off?
If it’s a one off then reply politely to her. If it’s part of a pattern of behaviour I’d suggest speaking to your partner about things. Maybe he needs to handle this. You could form a very firn response together for him to send to the ex with him also pointing out the inappropriate nature of her actions.

The 15 year old has two parents. Surely if they want their child to go on holiday they should be taking her.

Tell her mother to take her own daughter away, as you are doing with yours.

Quite.

Yanbu. The cheek of the ex getting involved too.

A bit to the bone. But yes, I have a 15 yo dd. They can be rather lippy and fragile.

Your dh can simply respond to his ex this is mother / daughter time paid for by the mother and without expectation that a sibling or step sibling should be included. He should also address it directly with his dd so she understands.

Ok thanks and sorry

Sorry, I was talking about the thread @KrisAkabusi was referring to, not this one.

It’s a birthday treat for DD and as such she gets to choose who she wants to accompany her. How is it mean spirited ?

Hi Ex, The trip is for DDs birthday present, it’s not a family trip, DH and DS are not going,it’s just me, DD and her bestie. Sorry DSD feels left out but I wouldn’t expect DD to be invited out when DSD spends her birthday with her friends.

Not according to the 3rd line in the OP I dont think

i thought the thread referred to was about a family holiday. OP’s DH wants to take his children (OP’s DSC) on holiday while their shared four year old stays at home.

Of course yanbu.

DSD is being treated the same as DD's other sibling, there's no favoritism.

This is a special treat for DD, not a whole family thing. Dad isn't going so clearly not appropriate for DSD to go.

Dad should do a nice day/weekend out with DSD and her own chosen best friend at some point in future.

I would tell her mother that she is more than welcome to take her daughter on a trip herself, however this is your mother / daughter trip. And as part of her birthday treat you've agreed to her friend going too.

yanbu. Your dsd could ask you both for a trip away with her friend for a birthday if that’s what she wants.
nothing wrong with taking your daughter and her friend if that’s what she’s asked for. S others have said, it’s not a family trip.

Does the mum know that the dad isn't going? It makes no sense that the mum expects the stepmum to take her daughter on a holiday that the dad isn't going on. Does she offer to take her daughter's half sister away with her?

This is a trip for the OP daughter and her, it is not a family holiday or the op's mum could take her on their own trip or the OP partner could arrange something

What a lovely present :)

YANBU. DSD is jealous due to the nature of the birthday present.

This is a birthday present, not a family holiday. No reason she should demand to go while your son stays at home.

To me, I find this comparible to some of my birthdays eg I remember a birthday trip to the cinema. My mum took me and my best friend from school, not me and my brother.

It’s also not really your problem about whether DSD gets the same kind of birthday present. That’s more up to her parents. It also doesn’t mean that her parents haven’t previously spent the same amount of money on her birthday before. A couple of nights in Rome through a deal doesn’t actually cost the earth.

It is a birthday treat for an 11 year old who will have a completely different experience with her friend who is the same age, than with a 15 year old.
DSD can have a birthday trip with her dad later and my guess is that she probably wouldn't want an 11 yr old tagging along on that.
The 11 year old's younger sibling isn't coming either.
I don't see any issue with this. I had a 10 year age gap between eldest and youngest and we did trips and holidays together and separately depending on the occasion.