What happened to Carrie
She was “stolen” by Blackwell as a pup. She was constantly fed Blackwell’s blood which, over time, started turning her into a vampire. Because she was a werewolf, she retained her wolfiness becoming basically a hybrid. Her hybrid wolf/vampire blood was then used to feed the vampires at Court so Blackwell could get on their good side and join them. No longer having access to Carrie, because of the MC’s and the Pack’s actions, Blackwell decided to kidnap the Pack’s kids with the intention of doing the same thing to them.
This is how I understood what happened to her.