>>I think this is your answer. She had a traumatic childhood and she's about to watch her dad start again with a baby that (hopefully) will have the loving and stable childhood she should have had.
Yep, all she had growing up, and even now, family wise, was her dad (no stable mum). She managed to get to adulthood accepting the way things were and at least, she thought, she had a unique place in her dad’s life, as his daughter, one thing that had lasted and she could count on. And perhaps she told herself that, at least, someday, when she had her own kids, she could build an intact, normal family unit with her dad as a conventional Grandpa (one whose relationship with young children is one generation removed, with all the traditional indulgences, treats, visits, and stuff, not altered by Dad simultaneously still parenting his own young kid). When you’ve had an odd, partial childhood, fixing things by having conventionality and giving a “normal” family setup to your own kids can be very strong and healing (speaking from experience here).
But that’s being taken away. He will have a new shiny daughter now, who is not only replacing her in the daughter role, but possibly also her own child to be, in the grandchild role.
Dad gets a do-over to give the new daughter all the stuff that the old daughter didn’t get (intact family, decent mom, etc). So many parents find this more interesting and fulfilling than their old kid that reminds them of the previous, less satisfactory family unit. And the old kid is expected to be super excited and about having their life now revolve around the new kid. Dad will be less willing to travel to her sphere now, too, I imagine, and she will be expected to fit into his new life, rather than him being an active part of hers.
Of course she feels sad and anxious at what she may lose.
if all of these thoughts make you angry because - of course it won’t be like that!! …good, because now you know what to do to help her. Be sure it isn’t like this. Consider her childhood and lack of mother, versus your intact family unit being built using the one piece she thought she had as a constant (her dad) and don’t make her feel like she should be grateful to be a bit player in a setup she never got to have.