*'m guessing this means that SD probably was not covered in glory either and the angry texts were on both sides.
Sounds like the Mum cannot do right in your eyes and there are issues of trust here. You tell here she cannot take her children on holiday as it clashes with yours (and you all booked at the same time) but she clearly feels the need to check this out with her daughter. Sounds as if frustration got the better of both of them, perhaps they have similar characters. Perhaps she was bitterly disappointed she is not allowed to take her children on holiday as you have priority (in your eyes). Perhaps she cannot usually afford big holidays?
I do think you need to step back and leave the first family members to it. It really is between them. I would suggest an appointment with a relate therapist (teenage specialist) between the DC and their Mum and that their father takes them and you are not involved. Costs about £30 for a session and will give all a chance to be heard.
Sounds to me as if the Mum is disapproved of by you and her ex and these feelings have influenced her children*
This is the most ridiculous thing I've ever read.
Sorry but the kids live with op. Op isn't telling their mum she can't take them, but ops holiday was booked first and their mum knew this. Only a moron would then book the same date. It's logistically impossible for them to go, it's not because op doesn't like their mum.
First family members? This is so insulting. I am not a 'first family member' and yet I do everything for my ss. Is it his mum making sure he's done his homework, feeding him, washing his clothes and generally parenting him? No. It's me. But I am clearly a second class citizen because I didn't give birth to him.
I would (and do actually) disapprove of someone who told their own child they didn't want anything to do with them.