This is devastating to her, because of the loss she feels.
The lost time and the different relationship her young stepsister is getting that she never got with this father.
I am sure she is also feeling the loss of her Daddy. He would be there with you to comfort her, but he is never going to be there again. She is at an age that Daddy's are important to their daughters. They enjoy being admired, praised and valued by the men in their lives. She is totally missing out on this. She has lot of love she want so to give and is not able to do this at this time. Her feelings are real.
I am not sure of the dynamics in her Fathers second family, but the cost, the timing and the person planning the trips has a lot to do with these situations.
The last 4 years means the stepmom is making sure he is being a dad to their child. She sees that he has not/does not give enough attention to his eldest daughter and is making sure that does not happen with their child.
He had a chance before he had his second child, to make amends with your child, but he totally dropped the ball and continues this same pattern. He is older and does not make the same mistakes, but cannot make up the neglect to your daughter (in his own mind).
What he does not realize is that
1. he needs to get past this guilt and make a plan to start over.
2. Your daughter is soon going to be going away to college and without a good relationship, it will become more and more of an effort for him to stay in her life.
3. Your daughter has her own life and is going to be busier than ever these last few years in High School and is not going to want to spend time with him over the fun school activities with her friends.
4. She needs him to validate her as a worthy person at this age. When she gets older she is going to realize he dropped the ball and is going to be very upset with him. The sooner he can get this relationship on a good track, the better for HIM.
How do I know all of this? I was a child of divorce and this is pretty much the same situation I had with my dad when I was a teen.
He was remarried and we did not see him very often. When we did it was with his wife and their family. He was working on keeping her happy. She also held the purse strings and so he had to "shuck and jive" to her demands.
It is hurtful, but as I got older I had a better understanding of the dynamics. I was too self conscious and not quite aware of what it was I wanted or needed from him. He HATED my mother so even though she tried to encourage him or give him a heads up, he would of the opposite of anything she told him or suggested.
And so the bottom line is communication.
Your daughter needs to figure out what she needs and wants.
And somehow this needs to get to her father. Either through your daughter. From you. Or with dad and daughter going to some counseling together.
Their situation is not like anyone elses. They need to learn to communicate with each other.
Give her a hug and remind her that parents are not perfect. They make mistakes. They are never trying to intentionally hurt their children, it sometimes just needs to be brought to their attention.