As a citizen, if you want to handle the entrusted matters as a client, you really need the client to personally confirm and submit the written entrustment procedures. Because you are not a lawyer, there is no written certificate (that is, official letter) issued by the law firm that the client entrusts a lawyer, and the court has no way to determine that your sister really entrusts you to help her handle it. The court is worried that if you don't examine whether your sister really means to entrust you, then after the execution, your sister will go back on her word and say that she didn't entrust you with this matter, and the court will be in big trouble.
Specific legal provisions are the internal trial rules of the court, not specific legal provisions, but basically closed internal reference guidelines.