The reason the Second Prince knew Fan Xian was a Prince was because when the Qing Emperor had summoned Fan Xian, the Crown Prince, and the Second Prince to dine together, before he left, the Qing Emperor had instructed the three brothers to enjoy the delicacies and hinted at Fan Xian's identity. Although Fan Xian had not understood the meaning at the time, the Crown Prince and Second Prince already knew that Fan Xian was the Qing Emperor's son. Thus, the Second Prince did not mention the specific details of Fan Xian being a Prince.

Yes, in Joy of Life, the Crown Prince knew that Fan Xian was a prince. In chapter 239 of the novel, the Qing Emperor asked Fan Xian and the Crown Prince to meet Lin Ruofu. In their conversation, the Crown Prince unintentionally revealed the truth that Fan Xian was a Prince.
The Second Prince knew that Fan Xian was a prince. In Joy of Life, the Qing Emperor once told the three brothers to enjoy the delicacies and hinted at Fan Xian's identity. Although Fan Xian had not understood the meaning behind it at the time, the Crown Prince and Second Prince already knew that Fan Xian was a Prince. Thus, it could be concluded that the Second Prince knew Fan Xian was a Prince.
The Second Prince knew that Fan Xian was a prince. During one meal, the Qing Emperor instructed the three brothers to enjoy the delicacies and hinted at Fan Xian's identity. Although the specific details had not been mentioned, the Crown Prince and Second Prince already knew that Fan Xian was the Qing Emperor's son. Thus, the Second Prince knew that Fan Xian was a prince.
When the Qing Emperor had gathered Fan Xian, the Crown Prince, and the Second Prince for a meal, the Second Prince had already known that Fan Xian was a Prince through the Qing Emperor's instructions and hints. The exact time was not mentioned.
The reason the Second Prince knew Fan Xian was a Prince was because when the Qing Emperor had summoned Fan Xian, the Crown Prince, and the Second Prince to dine together, before he left, the Qing Emperor had instructed the three brothers to enjoy the delicacies and hinted at Fan Xian's identity. Although Fan Xian had not understood the meaning at the time, the Crown Prince and Second Prince had already understood the Qing Emperor's meaning. Thus, it was certain that the Second Prince knew Fan Xian was a Prince.
The reason the Second Prince knew Fan Xian was a prince was that during a meal, the Qing Emperor had instructed the three brothers to enjoy the delicacies and hinted at Fan Xian's identity. Although Fan Xian had not understood the meaning behind it at the time, the Crown Prince and Second Prince already knew of Fan Xian's past. Thus, the Second Prince knew that Fan Xian was a prince.
The Second Prince knew that Fan Xian was a prince. When the Qing Emperor summoned Fan Xian, the Crown Prince, and the Second Prince to eat together, before he left, the Qing Emperor instructed the three brothers to enjoy the food and hinted at Fan Xian's identity. Although the specific details had not been mentioned, the Crown Prince and Second Prince already knew that Fan Xian was the Qing Emperor's son.
The Second Prince knew that Fan Xian was a prince. In Joy of Life, Fan Xian and the Second Prince were half-brothers. They were both children of the Qing Emperor. Although the exact time was not explicitly mentioned, Fan Xian's identity was when the Qing Emperor had gathered Fan Xian, the Crown Prince, and the Second Prince for a meal. Before the Qing Emperor left, he had instructed the three brothers to enjoy the food and hinted at Fan Xian's identity. Although Fan Xian had not understood the meaning behind it at the time, the Crown Prince and Second Prince already knew of Fan Xian's past. Thus, it was certain that the Second Prince knew Fan Xian was a Prince.
The Second Prince knew that Fan Xian was a prince. In " Celebrating Years," Fan Xian and the Second Prince were half-brothers. They were both children of the Qing Emperor. Although the exact time was not explicitly mentioned, Fan Xian's identity was when the Qing Emperor had gathered Fan Xian, the Crown Prince, and the Second Prince for a meal. Before the Qing Emperor left, he had instructed the three brothers to enjoy the food and hinted at Fan Xian's identity. Although Fan Xian had not understood the meaning behind it at the time, the Crown Prince and Second Prince already knew of Fan Xian's past. Thus, it was certain that the Second Prince knew Fan Xian was a Prince.
In Joy of Life, the people who knew Fan Xian was a prince were: - The Qing Emperor said," Fan Xian is the son of the Qing Emperor and Ye Qingmei, so the Qing Emperor knows that Fan Xian is a prince." - Chen Pingping was a trusted aide of the Qing Emperor. He knew that Fan Xian was the son of the Qing Emperor and Ye Qingmei. - Fan Jian was Fan Xian's father in name. He knew that Fan Xian was a prince. - Fei Jie was Fan Xian's teacher. He knew Fan Xian was a prince. - Wu Zhu was Fan Xian's servant. He knew Fan Xian was a prince. - Lin Wan 'er was Fan Xian's wife. She knew Fan Xian was a prince. - Si Lili was Fan Xian's confidante in Jingdou. She knew Fan Xian was a prince. - Yan Bingyun was Fan Xian's ally in Northern Qi. He knew Fan Xian was a prince.