Is there a chance with dp and his kids, that she’s feeling excluded from the family dynamic?
Especially if your dp only sees his kids evenings and weekend and they don’t experience the same level of discipline?
Plus the constant threat of sending her to her dads will add the exclusion and feeling of not belonging.
I was that teen and honestly it was the constant control by my mother. I felt enormous pressure to be “on” all the time, happy, smiling, nice, obedient. I wasn’t allowed opinions or thoughts, if i disagreed with anyone it was immediately me being “difficult” and my “hormones”. If i messed up in the slightest, said something that was taken the wrong way, it was immediate punishment and back to evil child status.
I couldn't keep it up. So i didn’t. I just excluded myself- easiest way was to kick off until i got excluded- Reverse psychology almost as if they thought i wanted to do something they would hold it over me...
They werr so set in the mindset i was difficult and moody they never looked at the big picture. Especially as i was an introvert with extrovert siblings who were always golden.
Take a step back and look at your dynamic. Do you treat dp’s kids the same? Are you tough on her? How would you feel if you were her.
Pick your battles. Don’t use the holiday as a threat- she will say she will go to her dads as that removes your power hold on her. Don’t make it conditional, focus on what a great time you’ll all have.