i am working in china,,, i want to invite my mother and sister for 20 days

Generally, your mother and sister should appear in person with required documents during the office hours of the visa office of China Embassy or consulate of their living place, and may answer questions from the visa officer. No appointment is required.

The required documents are as follows:
1. Duly completed Visa Application Form affixed with one recent color photo (30mm X 40mm).
2. Original passport with a validity of more than 6 months and minimum 2 blank visa pages, and photocopies of the first 2 pages of their passporta and recent Chinese visas if any.
3. If it is their first visit to China, local police character/clearance certificate is also needed.
4. Invitation letter from you and copy of your Chinese residence permit and the personal information page of your Pakistani passport.
5. original and photocopy of your sister and your birth certificates attested by Ministry of Foreign Affairs Islamabad.

After the submission, they wil be given a pick-up receipt for the visa application once it is completed. Finally, they can go to the embassy or consulate to collect the visa on the appointed day.