My sister and niece have S2 visa for a month. I want to extend their visa for 30 days. How to do?

Your sister and niece need to provide passport, passport-style photo, completed application form, valid registration form of temporary residence, an invitation letter from you and copy of your passport and Chinese residence permit, as well as your relationship certificate. And then go to the exit and entry administration of the local PSB to apply for the extension.