As his own biological sister, could she not mention it?
"What were you two even talking about? First, it was divorce, and then you suddenly brought Zhou Xing into the picture." Zhai Xin looked curiously at Zhai Yi and Zhai Yu. "Come on, talk to me too! Don't hide any secrets from me. We're triplets! Are you trying to leave me out?" That would not do. He would never agree to it.
"Nothing much." Seeing Zhai Xin's cheeky self, Zhai Yu clutched her forehead. She felt as though she could no longer continue with this conversation. She could not understand how Zhai Xin alone had turned out to be such a weird freak when the three of them were triplets, and both she and Zhai Yi had turned out just fine. Based on Zhai Xin's personality and character alone, no one would know that Zhai Xin was part of the Zhai family if he made no mention of his name. He was too immature and childish and behaved just like a monkey.