I could imagine an unknown Sandy Denny startling an audience but I can't imagine the Unthank sisters achieving that

Sandy Denny certainly startled audiences 40 years ago. Her clones (and I speak as one) did not.

I first encountered the Unthanks (and Jackie Oates for that matter) in the Bedford @ Sidmouth lunchtime sessions at around the turn of the millenium. I think you might reasonably describe this as a "folk club setting".

Most observers then had no doubt that all three (and others) would be "stars" in due course. Personally, I don't think either Winterset album does justice to any of the performers. They are far, far better live,

On the other hand, Jackie Oates' solo CD is a stunner.