To bring DD friend away instead of DSD

I don't think YABU but I can see why the 15 yo would be hurt and upset. She's a teenager and sees your DD getting an amazing birthday present and she's not and to rub salt in the wound a friend is also coming.

Maybe you should suggest your DH does something with her for her birthday? Sorry I've not RTFT so apologies if you've answered this.

It's your child's birthday treat, it's not like the whole family are going along as your youngest child is staying at home, so she's being very unreasonable.

It’s not a family holiday, it’s a birthday treat for your DD. Your DSD’s mother is being unreasonable.

Simple reply required here. “Because it’s dd’s birthday and she wishes to take her friend away”.’ If she questions this you can ask whether she’ll be inviting your dd to all her daughters birthday treats and to any trips she takes her daughter on. If she asks what dsd is getting for her birthday your reply can be, “I’m not sure, that’s between you and her father”.

DD's mother is bringing her away for her birthday. If SD's mother would like her DD to go on a weekend away let her organise it.
I'd just ignore it all now.

As for someone mentioning DD being brought on a "holiday of a lifetime" and SD being left out? It's a city break, weekend in Rome. Not a month in Dubai.

If SD's parents would like her to have a weekend away they should arrange to bring her. Not expect someone else to.

Do nothing until after the trip and then send her the details as she is free to arrange a trip with her DD just as you have with yours.

Is your DH father of both girls? What does DSD get for her birthdays? 11 isn't really a special birthday - are city breaks a standard birthday gift/treat in your family?

I can see why DSD is upset if her standard of birthday gift/treat isn't the same - she'll be 16 next birthday (a relatively special birthday) what plans are there for this?

My DC will have incomparable birthday treats/gifts this year. DS is getting city break for his 16th & DD1 is a leap day baby so last week she got a trip to the hair salon, a shopping trip & lunch out on her birthday along with gifts, balloons, cake etc DD2 doesn't have a special birthday this year so will get a standard birthday gift/treat within the regular birthday budget but when she was 16, 18 & 21 the other 2 didn't have special birthdays & she got extra special gifts/treats then while they got the standard birthday.

So I'm definitely not against special birthday treats/gifts but they do have to be fair & if this is a sudden change to the standard (or your DD always gets much nicer gifts/treats) then it's not really fair & your DH needs to be looking at that & doing something about it especially if he's father of both girls - if he's not then it's a bit more difficult but personally I'm not a fan of children in the same household being treated differently irrespective of parentage & what happens at the "other" house, as long as they are all treated fairly in mine.

Assuming that if your DH took dsd away for a trip to New York, dd and you would be fine about that? If so, crack on.
Is DS your dh's child only or yours too?

DSDs mum has messaged tonight to ask why is DD friend is coming instead of DSD

“Why? because it’s not a family holiday, it’s a trip for DD & friend.”

Perfectly reasonable. UNLESS dsd and dd are good friends. Then I think I would have offered the invite to dsd first. But if they aren’t particularly close it would be a bit odd to take her on your daughters birthday trip.

I wouldn't give this too much thought OP. It's a birthday treat, your youngest and DH aren't going, it's none of the ExW business. I'd simply say to DSD, well, you best talk to your dad about what you want this year and smile. I bet a zillion pounds that if she wants a similar trip she will NOT want 11yo tagging along.

The trip really isn't anything to do with dsd. Tell her it isn't personal, and not a family trip. Mother and daughter & bff trip not two sisters.

Her own dm can take her and her bff away. They have a nerve even thinking dsd can invite herself.

For the same reason her younger sibling is not coming - it's a birthday treat, not a family outing.

‘It’s a treat for DD and she wanted a friend to come.’ End of. Don’t reply to SDs mother, that’s for your husband to sort. Not unreasonable at all. You are allowed to treat your own child. If your SD’s mother wants to treat her, she can open her own purse.

What did DSD do for her 11th birthday?

I can see why she's upset, even if she's not being totally reasonable.

It's so shit being in a blended family - this thread is reason number 12,647

If it was just you an DD I'd think it was fine but to take another kid before considering immediate family is a bit off imo.

If we were doing this with one of ours, we wouldn’t expect eg DS2 to ‘take’ DS1 as his plus one, he could take a mate. It’s a gift, not a family holiday

Yes, totally different. This is a gift to the child - she can take her mate if she wants, rather than a sibling, whether full or half.

He is not ‘minding’ the LO! He is parenting his child, it’s not doing OP a favour. He’s looking after his own child.

It's a birthday present not a family holiday. Yanbu

It depends on whether DSD ever gets to go away, either with her mum or dad, I think.

A trip of a lifetime?! It’s a few days in Rome!

YANBU. Of course you can take your daughter (and her friend) on holiday. I actually think it's a bit odd that you're paying and DH isn't contributing to his DD's birthday present.

I think you can tell the ex that your other child isn't coming either - it's just for DD. Then your DH can tell her that on any future family trips, your DSD will be invited. Hopefully that shifts the communication around this onto him.

Just tell her its a birthday gift and DD picked her plus 1 🤷🏻‍♀️

This isn't your family holiday it's a birthday present to your dd. If she wanted to take a friend to the cinema or bowling would that friend always have to be dsd?

I don't like to see dsd being treated unfairly although I think with holidays providing if they get a holiday with at least one parent they shouldn't necessarily have to do all holidays with both.

But in this case it's not even that it's not a family holiday it's a trip for dd and a friend for her birthday. Your dh could always do the same for dsd birthday.