Ferdinand of Aragon wanted his younger grandson and namesake Ferdinand, born and educated in Spain, and not his older brother Charles (born and educated in the Netherlands) as heir of the Spanish kingdoms. Maybe he lives longer and Emperor Maximilian die before him so Charles is already the Emperor and the King is able to persuade the cortes of Castile and Aragon to make Ferdinand his heir. What will happen after that?
Ferdinand VIII (i think) is the ruler of the Spanish empire and has with him his youngest sister, Catalina (also spanish for birth and education), while Emperor Charles V is the ruler of the Empire, Austrian lands and Netherlands and their other sister (Eleanor, Mary and Isabella) also live in Netherlands.
What will happen after? Emperor Maximilian signed a treaty with the King of Bohemia and Hungary for a double marriage so Mary (as OTL) or one of her older sisters will marry the future Louis II of Hungary and Charles will marry Anne of Hungary (his brother OTL wife). Also for better securing the succession of young Ferdinand is likely who Ferdinand of Aragon will arrange a double wedding between his grandchildren with his son-in-law the King of Portugal so Catalina will marry the future John III as OTL and Ferdinand the eldest daughter, Isabella (who in OTL married Charles).