Harry and Tavi are said to be part of the same bloodline, not necessarily descended from the same person. That heritage bestows something important. Like how Constantine had a heritage that allowed him to face epic level magical threats.
It is not the cops are but local multiversal region, I would say it would be akin to the else where's of the dc multiverse. But instead of based on the same world, the else where's are are different what ifs while these are entirely new worlds.
Harry has said the never never potentially allows access to alien worlds, so those different worlds could potentially be in the same universe. Nothing we have seen has definitely made it know that these are differing worlds, just that these humans are no longer on earth, and their magics have gone another path.