The correct choice is the first one: “On the right – you're Diona!” If you choose the correct answer, you'll only have to fight Diona. If you choose wrong, the real cultist will attack you. They can be defeated easily, but you'll feel the ramifications of killing the innocent sister.

Which twin is the real Diona?

Which One is Diona? As you progress through Diona's quest, you will eventually get to a point where you must choose who is actually Diona. The correct choice is the twin on the right.

Did Kytheras leader kill the son?

A man will be interrogating another about the murder of his son. ... You can accuse whomever, but the real murderer is the Leader of Kythera. He'll then ask you to kill the leader to avenge his son. Do so to complete the quest.