Grindelwald had to have lied about being brothers, there could be a relation but I doubt it. It seems clear to me that the reason he went after Credence in the first place is you use him as his tool to destroy Dumbledore. Credence only wanted one thing, to know who is family was. The kid is also at his most powerful when angry and scared.. Telling Credence he has a brother who could take care of him but did not would fester and build hatred, resentment..Actually all Grindelwald did is tell the story of what happened to Arianna just omitting her fate. Nor do I think that it would take much to fool Credence with magic, he does not seem to know much. Lets not forget Grindelwald is as powerful as Dumbledore, creating a Phoenix to sell his tale, would be easy, even getting it to stay as long as Grindelwald is alive not that far fetched either...
