I have two mates and one will die or I die

Alexis Grey is a part of the Redsun pack her father is alpha until her older brother Kayden takes his position in 6 months after he’s married and fully mated to his mate Amber. Alexis is sixteen and still yet to shift into her wolf. Normally wolfs will shift on their sixteenth birthday but for some reason she didnt however she did meet her mate, little did she know that she has two mates who are both alphas , having two mates has never heard of until one day her father came across a book that speaks of a she wolf that is born every 3000 years that can control every single wolf and human out there, She cannot shift and be able to start using her powers until she is fully mated and has her first born child, She has to mate with both of them in which one of her mates will die the mate that lives is her true mate, if she does not mate with both she will die herself , if she dies the whole population of Werewolf’s will be in danger because of the curse that is unleashed when she takes her last breath of life.