First thing first, the castle needed to prevent Sarah from catching-up with her beloved Nate. So the haunted building used its power to rearrange its rooms, making the werewolf go head first into another room.
At first, Sarah thought she had entered an exceptionally big broom closet. How else could she explain the pile of brooms she ended in? But as she navigated around the room, she noted the shelves of vials, most of which were full of some strange potions. Then, there was this cauldron in the room's fireplace. “I'm in a whitch's labs!” Said an exited Sarah, momentarily forgetting Nate.
The haunted castle too was exited, but not for the same reason. Sarah had walked in its mistress laboratory, meaning the accursed werewolf would soon cease to be, replaced by its resurrected creator. But in its long solitude, the castle had forgotten some of the rules of its own magic, like the fact it couldn't undo what it had done.
So while Sarah did gain the powers of the castle's former mistress, knowledge include, she didn't became less of a werewolf, nor did her personality or memories were altered.
Armed with the powers (and the knowledge) to act before the castle changed the one she love, she teleported out of her lab and emerged just behind Nate, looking like an ordinary wolf. She jumped at him, biting the first part of him she could: his right hand.
Then, Sarah changed back to her almost human form. Crying, she said how sorry she was to have rejected Nate, how she wouldn't have if she hadn't be under the castle mental influence at the time, before confessing her love for him. Since she was both a witch and a werewolf, she knew her pack didn't work exacly like a pack of ordinary wolf.
Unlike she first thought, even if Nate become her mate, her pack wasn't going to be led by a pair or alphas, but by a lone one: herself. Which mean, to Nate, she will always be his alpha first. But as long as he was her mate, she didn't care, so once he had finished his transformation, she teleported them to the castle's master bedroom, HER bedroom, and initiated the mating ritual, forgeting about Mickey and Cody in the process.