My issue isn't even with Jordan as much as it is with Miles Teller and Jamie Bell. While having a black Johnny but a white Sue makes no sense whatsoever and feels like stunt casting, at least those two can fit the roles. Miles Teller however is a short, flabby nerd and Jamie Bell a lean, slim Brit. How these two are supposed to convincingly play a brilliant super genius and a big old school New York street brawler is beyond me. Now that I think about it, with the added rumors of a female Dr Doom, this whole movie is starting to feel like random casting for the sake of random casting.
They're being inspired by Ultimate FF, so Reed is only 21 in that and obviously it's a different interpretation. Jamie can put on weight, that's nothing.
As far as Michael B Jordan and Kate being related, that can easily be explained of them either being adopted brother and sister OR step brother and step sister. I don't think its stunt casting as much as its Josh Trank likes working with Jordan, likes him as an actor and feels like he can play the part and wanted him to do it