Complex sentence: However it was later that Juana's relatives' heirless deaths made Juana the great heiress she turned out to be: so important an heiress that actually her husband gained opportunity to be threat to the royal power.
Split into simpler sentences: However it was later that Juana's relatives' heirless deaths made Juana the great heiress she turned out to be. So important an heiress that actually her husband gained opportunity to be threat to the royal power.