Leave. It would be one thing if she truly wanted a family with you, and was willing to prove it by keeping her promise. What on earth is wrong with Alex having a half-sibling? What? Nothing at all, except that they're more concerned about building their bond, and have been from the beginning, than with being even a little fair to you.
She has lied to you over and over and over, both of them have, and now they have, together, without your input, made a decision that will have major hurtful consequences to you. You deserve better than this. If they won't respect you, then it's up to you to respect yourself to walk away, or else you will become bitter over time, and poison your own life as well as theirs. You CAN find love with someone new, so many others who have left a long-term relationship have. You're going to be ok.
I'm so sorry. :/
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.