From Shoujo Manga Maniac:

Duan Cheng Feng's aunt is one of the emperor’s concubines. In order to keep her high ranking status, she gives the emperor a child, but to her dismay, it’s a girl! She tries to convince her family to exchange the female baby for their nephew. In order to avoid it, the young boy’s parents fake a serious illness to make her give up her plan. But she won’t let a minor detail like that get in her way...