How were Isabella I of Castile and Ferdinand II of Aragon related?

Ferdinand II of Aragon was the second cousin of Isabella I of Castile.

Their nearest shared ancestors were John I of Castile and Leon and Eleanor of Aragon, Queen of Castile.

That is only the shortest of 2 separate lines of descent between them.

The line between them

1. Isabella I of Castile 1451–1504
2. John II of Castile 1405–1454
3. Enrique III of Castile 1379–1407
4. John I of Castile and Leon 1358–1390
5. Ferdinand I of Aragon 1380–1416
6. John II of Aragon 1398–1479
7. Ferdinand II of Aragon 1452–1516

See the line on the tree