I always find threads like this fascinating.
If OPs DD was a step daughter everyone would be defending her as the first child.
What she said was unpleasant yes, and shouldn't have been said but she's a teen who probably doesn't even understand how and why she feels that way, well enough to be able to articulate it.
Having been a teen when both of my parents, separately, went on to have further children, it's not all roses. I love my little sibling now but at the time the resentment was high, in fact at times it still is.
He wasn't a "sibling" he was a child my parent and their partner decided to have, he became the reason people missed my parents evenings, that i had to move bedrooms, when they were already struggling (less than I was younger but still) I had even less when actually I needed more...I needed new clothes and hygiene products and to have time spent on me (as did my elder siblings) instead we watched us get pushed to the side in our key years (GCSEs for me and my sister) and him get lavished with everything we wanted and deserved.
Even now as all adults, he's like the prodigal son and we try to live up to him.
It's not easy being the outsider, but it's even less easy to communicate how you feel when everyone else is excited and happy.
So you need to separate the baby issue and the general disrespect (throwing clothes after a couple of wears?! At least sell them) and deal with them as separate things.