Wow... so for all that time, as far as she is concerned, it's been you and her..
And right now, when she's at her probably most hormonal, most insecure, most shit going on she doesn't really fully know how to handle, when she doesn't quite grasp how she fits into the world or where she stands wtih a lot of people around her...
You tell her loud and clear...
I have a new partner - you are not enough for me.
We, new partner and I, not you, are having a new baby. You are not enough for me.
As far as she is concerned, your actions speak volumes, what you have DONE here is what is meaningful, so whatever it is you say right now, its all just fluff, its meaningless, its trying to sweeten the pill but it isn't true. If it were, you'd not be starting a new family, one to whom she is superfluous at best and frankly an unwanted pain in the arse and in the way at worst.
This is monumentally shitty timing on your part, if you could have hung on for another two or three years it would have been signifcantly better.
Talk to her, but listen, don't talk AT her.
Reassure her, but don't tell her lies, for example, you can't tell her she's your priority, she isn't and she won't be. You can't tell her she always comes first... she won't.
She's 14, she is immature, insecure, inexperienced, but not stupid.
Actions speak louder than words... show her you are making time just for her, don't just tell her that you will. Actually do it and if she wants to sit and ignore you, thats up to her.
If she does want to investigate living with her father and that is seriously an option, allow her to explore that. Yes, there is a chance she will bog off, not speak to you, decide for herself that you didn't want her - but the other option is that you refuse to allow this AND she ends up feeling like she's had an enforced shitty time as the relegated older half sister. Neither is good!