The cube within Xiao was only guiding certain actions of Xiao and blinding him, like a cover restricting him from understanding some things. It was as if his sharpness and his ability to see around him had been masked over, hence, not all actions Xiao took were out of control beyond him.
He made a 'connection' with his sister because he indeed wanted to share her pain and get close to her as her brother. He wanted to make up for the useless brother he was. He did talk and help Yurou because he considered her a good child, an innocent little girl who stuck to his side even when the rest of his family was avoiding him.
He helped Li Lun because he owed her a favor for saving his life. Plus, after reading through the Heavenly Opposers, he understood the life she had lived, which had somehow connected with his, thus making him want to help her.
