Their Hells Bells.

Isabella Swan had a past that none of the Cullen's nor the pack knew about. One day Bella flipped. Bella had some mental issues. She had bipolar and bad anxiety along with PTSD. Now She had friends that were known to the supernatural but they weren't supernatural themselves...Well not all the time. See Dean and Sam Winchester are supernatural hunters. They hunt the bad and kill them. See they are very good at what they do. However once Bella turned 18 Dean and Sam were right there to get her however at this point Edward's back and it seems like they are having a fight with the pack. And guess who comes along...Dean and Sam....I mean she is their Hells Bells for a reason.