Documents needed for multiple entry visa

You may need a S2 visa for visiting your sister. And the requirements for a S2 visa from the official are:
An invitation letter issued by the inviting individual (a foreigner who stays or resides in China for work or studies) which contains:
1. Information on the applicant (full name, gender, date of birth, etc.)
2. Information on the visit (purpose of visit, arrival and departure dates, place(s) to be visited, relations between the applicant and the inviting individual, financial source for expenditures, etc.)
3. Information on the inviting individual (name, contact telephone number, address, signature, etc.)
A photocopy of the inviting individual's (a foreigner who stays or lives in China for work or studies) passport and residence permit
Photocopy of certification (marriage certificate, birth certificate or notarized certification of kinship) showing the relationship of family members between the applicant and the inviting individual.
The flight ticket is not required and you can book after you get your visa.