To get the achievement, you will need to play as Castile (recommended) and wait until the Iberian Wedding event kicks in.
You need to choose "Let us bind their dynasty to ours"(picture is outdated but just click it)
since Aragon almost always still keeps their PU with Naples, this would work. If Naples becomes free i recommend starting over.

When you have clicked the button, you should get the achievement.

PS: Iberian Wedding kicks early in the game so it's a quick one.