"Oh no it's fine it just feels a little weird seeing my sister with you and you get the idea", the very idea that Aiden had been made a thrall meant that he wouldn't be able to include him in his plot against Isabella. Even worse if he was free and genuinely had feelings for her it would possibly make Aiden hate him once the deed was done. He figured for now it was best to exclude him from the plans and let everything unfold.