I believe in something called “Kin by Extension”, this basically means that you are Kin with something but not directly. Examples of this could be alternate forms (Luna and Nightmare Moon) or Fusions (Pearl and Amethyst). Would you say that this is a “valid” belief?

Never heard of it. But if you’re kin of someone that has multiple elements to their self, like a fusion, then yes, you are probably also kin with their separate elements. There are obviously cases where this would not be true, but in general I would say it would be illogical for it not to be the case.