I do not guarantee that this will work in your particular set of circumstances, but it did the trick for a similar save I created to test it:
In this order:
traits.equip_trait trait_WerewolfPackB
traits.equip_trait trait_WerewolfPackB_Member
Then take the sim to the trailer at the Wildfangs hangout. If it works, that sim should be able to claim pack leadership by clicking on it.
If you don't want that sim to remain alpha of the Wildfangs, once you claim alpha you can entrust leadership to another, such as a "resurrected" Rory (once you've made her a pack member.)