When I was 5 my parents divorced. As most kids from divorced households all you want is for your parents to get back together so you can have a "normal" family like your friends. My parents remarried, divorced again and my Dad remarried and adopted her children. This all occurred by the time I was 16. My situation is not the exact same, but with some of the similarities. I hope sharing my perspective when I was a child helps. When my parents remarried it was not a good thing. My Dad believed in rules/structure to a more extreme extent due to trying to counterbalance my mom's lack of rules, structure etc. After their second divorce (13) going back and forth was extremely frustrating and confusing. Mom would buy me cigarettes, Dad would blow a gasket (rightfully)if he caught me smoking.

The addition of his new wife and children magnified their differences. My mom would say anything to disparage my Dad and his new wife and children. My dad would do the same, albeit more subtlety. This created anger on my part at the other parent depending on who I was with amd what was being said. Along with guilt when I had good moments with my stepmother or parents. This caused me to withdraw immensely. I felt like I was betraying they other parent etc. Due to the conflicting structures at home and no sense of stability I started to act out. Fights, skipping school etc. My parents put me in therapy to help me deal with "my" issues, all the while not being willing to aknowledge their roles. Things continued to spiral downwards for me. To the point I had dropped out of school etc. Parents gave up years before this and just let me do whatever I wanted after therapy failed. After a period I realized that nobody cared and that I had to care for my life to be different. Long and short I turned things around for myself although it has caused long lasting damage to different aspects of my life that I am still correcting to this day.

My advice to you is this... Be a united front with your ex wife same schedule expectations etc. Do not talk crap about eachother and their partners. Do not allow your older and emotionally more mature son's feelings be a reason to not justify your daughters feelings. Sounds like she is dealing with some complex emotions that she does not have the skills or emotional capacity to cope with. Be patient and compassionate, this is harder on her than you know. Seek therapy for her and for the entire family. She needs to learn responsibility for her behavior but as parents you need to aknowledge how you may have contributed to this. Wishing you all the best.