Still unable to start fight, other sisters have been defeated, completed other zone arcs.. Seems there will be no way to reset this and will require an update from Ubisoft to resolve unfortunately. Huge bummer wanted to finally get into a new set of bear armor.
I'm unable to start the fight at @ Walsham Crag - there is nothing to interact with to start the fight. This occurred after 5 or so boss attempts - I've moved on hoping the quest would reset but it looks like it's still broken.