But she was the heir IOTL.
She's only the heir after those before her are dead.
http://fmg.ac/Projects/MedLands/ARAGON%20&%20CATALONIA.htm#_Toc203441670
King Fernando II & his first wife had six children:
1.
Infanta doña ISABEL de Castilla y Aragón (Duenas 2 Oct 1470-Zaragoza 23 Aug 1498, bur Toledo). Susceptible to melancholy, probably inherited from her maternal grandmother, she experienced a period of profound depression and religious fervour after the death of her first husband. She was recognised as heir to the throne of Castile 29 Apr 1498, and declared Pss de Asturias.
m firstly (by proxy Seville 18 Apr 1490, in person Lisbon 22 Nov 1490)
Infante dom AFONSO de Portugal, son of JOÃO II King of Portugal & his wife Infanta dona Leonor de Portugal (Lisbon 18 May 1475-near Santarem 13 Jul 1491, bur Batalla). He died after falling from his horse.
m secondly (Valencia de Alcántara 30 Sep 1497) as his first wife,
MANOEL I King of Portugal, son of Infante dom FERNANDO de Portugal Duque de Viseu & his wife Infanta dona Brites de Portugal (Alcocheta, Tago 1 Jun 1469-Lisbon, Royal Palace of Rive 13 Dec 1521, bur Belem, Monastery of São Geronimo). Infanta Isabel & her second husband had one child:
a) Infante dom MIGUEL da Paz de Portugal (Zaragoza 23 Aug 1498-Gránada 19 Jul 1500, bur Zaragoza). Principe de Asturias from birth, as heir to the throne of Spain.
2.
Infante don JUAN de Castilla y Aragón (Seville 30 Jun 1478-Salamanca 4 Oct 1497). Principe de Asturias.
m (Burgos 3 Apr 1497) as her first husband,
MARGARETA Adss of Austria, daughter of Emperor MAXIMILIAN I Archduke of Austria & his wife Marie Dss de Bourgogne (Brussels 10 Jan 1480-Mechelen 30 Nov 1530). Infante Juan & his wife had one child:
a) daughter (b and d Alcalá de Henares early 1498).
3.
Infanta doña JUANA de Castilla y Aragón (Toledo 6 Nov 1479-Tordesillas 11 Apr 1555). Lived in Flanders with her husband. She replaced her nephew Miguel Infante de Portugal as heir to the throne on the former's death in 1500, returning to Spain briefly from Mar-Dec 1502. She succeeded her mother in 1504 as
JUANA “la Loca”
Queen of Castile, arriving in Spain 26 Apr 1506. Her father left the Government of Spain to her husband, retiring to Naples. Having shown signs of mental instability from an early age, Queen Juana descended into insanity after her husband's death, and was restrained at Tordesillas from 14 Sep 1509 for the rest of her life. She succeeded her father in 1516 as Queen of Aragon, but the Cortes accepted the succession of her son Carlos 7 Feb 1518, on condition of his abdication should his mother recover her reason. A brief insurgence in her favour took place Aug/Sep 1519, but it was crushed by King Carlos.
m (Lille 20 Oct 1496)
PHILIPP Archduke of Austria, son of Emperor MAXIMILIAN I Archduke of Austria & his wife Marie Dss de Bourgogne (Bruges 22 Jul 1478-Burgos 25 Sep 1506). He is sometimes referred to as
FELIPE I King of Castile.
-
KINGS of SPAIN.
King Fernando II & his second wife had one child:
7. Infante don JUAN de Aragón (b and Valladolid 3 May 1509). Principe de Gerona.