This was the new plan.

Aro decided that he should let the pair go, he should stop arguing with them.
He should have guessed that prohibiting Didyme from leaving would stir her rebellious nature and he should have seen that Marcus was still blinded by her. There was a solution to the fights and misunderstandings and that was letting them go. Because Aro knew they would come crawling back soon enough, disheartened but enlightened as to who they truly are.

Because Aro knows everyone pretends and everyone lies and cannot quite escape their nature. He keeps telling himself that he isn’t desperate and hopeless but he knows it isn’t true. Vampires don’t change and he will forever be doomed to carelessness caused by his heart’s frantic search. And so, Marcus will forever be doomed to a leading role, that he supposedly despises so. As if he would ever be sated by anything else.
He began his quest of building an empire on a petty, little whim, as if he could destroy the idea of royalty by creating his own. An ambitious, idealist nineteen year old who wants to rule and thinks he can do better than everyone who came before him. Aro had seen through the flimsy lie that very first day they met. And he has been proven correct ever since. Marcus seeked out a Roman general after all, someone who knew how to enforce rules, how to keep people in order, someone who valued laws. Making guards, taking down other covens, helping Aro himself when he made that deal with Kleo. Is there a more certain way to establish power than having a god on your side?

And then Didyme, who as opposed to him, can never be called impulsive in her actions, had to fall for him. She was always so viciously entranced by the idea of freedom, she had nothing to prove, no carved path to stray from and of course, that attracted Marcus. She was someone he could not rule. Aro saw that Marcus thought, or tried to believe, that part of his love for her was his desire to be like her, unbound and untamed but Aro knew better. Marcus was almost obsessed with her because his need to control was undying and she was resisting him.

He predicted that if Marcus left with Didyme, the two would be happy for a few years, thinking they somehow deceived fate, or maybe even Aro himself. But then Marcus’ skin would start to itch and one day he look around and he would see that he has gathered himself a new flock to lead. Servants perhaps, founded towns or worst of all, he will manage to enslave Didyme and break both their hearts.
And when that happens, the pair will return to the palace, disillusioned with themselves and finally knowing perfectly well where their place is.

He went to find his sister, talk to her about having a change of heart, and that she absolutely should run away with her soulmate.