Well, is he???? MT has gone as far as she possibly can with this idea, Fritz. And no, she won't accept any younger Hohenzollern siblings for her boy. No one less than the King would do! Joseph is the future Emperor, after all.

(And already King of Germany; Franz and he did go through the relevant ceremony in Frankfurt which would ensure that Joseph after his father's death would automatically become Emperor instead of having to go through vote gathering all over again. So he's already outranking Fritz, technically speaking.)

(In medieval times, it went thusly in the Holy Roman Empire: the princes electors voted you King of Germany, you got crowned as same, and then the Pope made you Emperor, unless you were one of those kings he was busy arguing with. Emperors who wanted to ensure there was not too much bother about the succession had the princes electors vote their oldest sons as Kings of Germany while they, the Emperors, were still alive and well. In the sunset days of the HRE, people usually didn't bother anymore, but after all the trouble with her own succession, MT wasn't taking any chances. She had Joseph made King in his father's lifetime.)