What if Isabella, Juana, Maria and Catalina had been born as boys and Juan had been born as a girl? This gives the Trastamara monarchs four sons and one daughter instead of four daughters and one son. Who would be good marriage options?
OTL Isabella would likely be named as Juan
OTL Juan would likely be named as Isabella
After that, let's say Juana is instead Ferdinand, Maria is instead Alfonso, and Catalina is instead Pedro.