I'll return to that idea with some modifications: John Albert and Alexander die as kids, not long after birth of their youngest brother Frederick. So compared to previous version I'm expecting few changes:
-Vladislaus may be under stronger pressure to consumate his first marriage with Barbara Hohenzollern, say he did it. He had son (Charles, named after his great-great-grandfather, Charles of Luxembourg, who was remembered as great king in Bohemia) born in 1481, and Elizabeth, born in 1483. Barbara dies giving birth to Elizabeth. During struggle for Hungarian throne Vladislaus is facing less problems-not only he don't need to compete with younger brother, he could also marry Beatrice without OTL schemings and would not bother to get annulment if he has heirs already.
Meanwhile Sigismund, who is obvious heir of his father after death of Saint Casimir, could marry Kunigunde of Austria, who was previously proposed to his pious brother.
Frederick Jagiellon would not join the Church, after death of brothers he'll be needed as spare heir and likely ends as Grand Duke of Lithuania (and marry Helen of Moscow) while Sigismund gets Poland.