@ojss21
I think it's the car seat comment that's upset me the most. Surely even at 15 you understand the difference between a safety item that is a necessity for a baby and £100 birthday cash? I wish I could have afforded to give her £250 birthday cash, I honestly do. But on mat leave pay I just can't
It's not about the car seat per se, OP, but what it represents. She feels like she's losing out to the new baby, not just in money, but in your time, attention and love. I'm sure she understands that a car seat is essential, but that £250 car seat has become an emblem of all that she's lost now she has a new sibling. She's no longer uppermost in your mind.
She's been the most important person in your life for all of her life, and now she's not. It's probably made her feel insecure and a bit destabilised, and at a time when she's ill-equipped to cope with it.
Teens tend to be highly emotive and reactive, they're awash with hormones. Plus the temporal lobe of the brain, which is linked to emotions, tends to develop more in the teen years and it takes a while for the bits that enable them to be logical and to control their emotional responses take a while to catch up.
I'd try a bit of love bombing for a while. And making sure that she knows she's foremost in your mind, consult her when making decisions about family stuff, like your trip.
With the visit to your sister, explain and apologise as a pp suggested. Would it be feasible for DD to come home by train, so she can go out with her friends as planned? If so, offer her that choice.
Your posts really resonated with me, OP. I was on the brink of puberty when my brother was born, and more than 50 years later I can still recall how awful I felt. Even a slightly askance look from my mother would hurt and made me angry. It was a dreadful time, and I carried a huge sense of grievance for years afterwards.
It lasted well into my 50s. When my late DF was in his final illness, I went to see him in hospital. He didn't recognise me, and I put this down to his dementia. Then my brother arrived, and on seeing him my father's face lit up. I felt a stab of pain and rejection at the lack of recognition, just like I felt for years in my teens and I was 54! The shock took my breath away, quite literally. And it still hurts now, just thinking about it, more than 11 years later.
Be as kind to her as you possibly can, and try and remember that when she lashes out, it's probably because she's hurting.