Basically, yes, I think so.
You could always run the argument that the new arrangement with the sibling/neice/nephew is not registrable because it's not a "tenancy" but rather e.g. a caretaker's agreement that lacks any of the hallmarks of a tenancy. But if it comes to a row the Tribunal is unlikely to be sympathetic to this argument, so I wouldn't rely on it unless I was prepared to take it to court.
You could always run the argument that the new arrangement with the sibling/neice/nephew is not registrable because it's not a "tenancy" but rather e.g. a caretaker's agreement that lacks any of the hallmarks of a tenancy. But if it comes to a row the Tribunal is unlikely to be sympathetic to this argument, so I wouldn't rely on it unless I was prepared to take it to court.