Why does my brother need two kidneys?

In "Beautiful Lies", when the younger sister didn't have one kidney, the younger sister originally had two. Later, before she died in a car accident, she gave her kidney to her seriously ill brother.

The reason why my brother needed a kidney and the final result:

My ordinary brother Fanfan was diagnosed with uremia. Only a kidney transplant can save Fanfan's life. Cheng Gang and Sun Yan both rushed to donate a kidney to Fanfan, but Sun Yan stopped Cheng Gang from donating a kidney. Sun Yan forced Pingping to donate a kidney, but Pingping was angered by her mother's behavior and left home. Although Fanfan was seriously ill, he still pretended to be strong.

Cheng Gang brought Cheng Dan with him to persuade Pingping to donate a kidney, but Pingping still refused because of fear. Xiang Xiaowei sent a text message to Fanfan to express his love. Fanfan was in a good mood on the hospital bed. Cheng's mother did not hesitate to use her own life to awaken Pingping, and Pingping agreed to donate a kidney to Fanfan. Seeing that Fanfan was weak, Pingping cried for his brother with tears in his eyes. Sun Yan and Cheng Gang were overjoyed when they heard that Ping Ping's matching was successful. The surgery was about to take place, but Ping Ping ran away before the surgery because of fear.

Fanfan's condition suddenly worsened, and the doctor told Sun Yan and others that Fanfan might not be able to wait for surgery. Pingping rushed to the hospital to find Fanfan, but she died in a car accident on the way. Sun Yan reluctantly signed the contract, and her mediocre kidney was transplanted into Fanfan. Looking at Ping Ping's cold body, Cheng Gang reluctantly sent his daughter on her last journey. Fanfan recovered from his illness, but his family concealed Pingping's death out of fear that he would be sad.