How can i call my sister and brother in law to china i am on Z Visa? Which letter needed?

You can issue an invitation letter to your sister and brother-in-law, and then they can apply for an S2 type to enter PRC to visit you. Aside the invitation letter, you also need to provide copies of your passport and residence permit. Your sister and brother-in-law need to provide application form, passport-style photos, passport and its copy, and your relationship certificate.