Why is Cheng willing to help Gu and Su reconcile?

Because they have recognized the relationship between brother and sister.

In the play, Su, a lively and clever flower girl who loves life, and Gu Fu, a cool and handsome gentleman. In order to earn medical expenses for treating his younger brother's leg disease, Su mistook Gu Ziqian, the second young master of Gufu, as a "little xianggong" when he was a child, and entered Gufu as a maid. Mrs. Gu accidentally mistook Sue for Miss Gu Waner who had been missing for many years. Gu Wei made a contract with Sue and became "brother and sister".

The short story goes like this. Su Xiaoxiao, a flower girl who is good at cheating, mistakenly entered into a "brother-sister contract" with Gu, the eldest son of Gu Fu, a jewel family, thus becoming Gu Waner, Gu Wanxi's sister and family wife. However, the story of a sparrow changing into a phoenix is not as beautiful as Su imagined. In the face of this vicious "brother", Xiao Ye can only follow suit and fight bravely. Although the journey is hard, fortunately, there is a warm-hearted second brother, Gu Ziqian, who has been accompanying my grandfather and giving her meticulous care. But this second brother's care for my grandfather seems to be more than just the love between brother and sister. To make matters worse, in the process of getting along with his two brothers, Xiao Ye found himself getting deeper and deeper, and fell in love with his most annoying eldest brother Gu.