Once I-130 for your sister is approved by USCIS, they will transfer your case to NVC (they will send you notice of action for approval and will also mention that case transferred to NVC).....after this NVC will assign case no...........you need to call NVC and get that case no.
next thing you need to do is to give NVC email addresses of you and your sister.
Choose petitioner (yourself) as CHOICE OF AGENT - DS 3032.
Steps once NVC has your case:
1 Assigns a case number to the petition.
2 Send Form DS-3032, Choice of Address and Agent, to the applicant and the Affidavit of Support processing fee bill to the petitioner.
3 After the applicant selects an agent, the NVC will send the immigrant visa processing fee bill to the agent. Note: The agent does not need to be an attorney.
4 After the immigrant visa application processing fee has been paid, the NVC will send instructions to the agent, which explain the Applicant Document Process.
5 After the Applicant documents are submitted to the NVC, the NVC will review the information submitted for technical correctness and completeness.
6 After the Affidavit of Support processing fee is paid, the NVC will send an instruction letter to the petitioner, which explains the Affidavit of Support Process.
7 After the Affidavit of Support documents are submitted to the NVC, the NVC will review the information submitted for technical correctness and completeness.
8 After reviewing the submitted documentation, and the file is complete with all the required documents, the NVC will send the petition to the embassy or consulate where the applicant will apply for a visa when the case file is complete. For certain embassies/consulates, the NVC will schedule the applicant's interview. Approximately one month before the applicant's scheduled interview appointment with a consular officer, all interested parties (applicant, petitioner, and third-party agent, if applicable) will receive an appointment letter containing the date and time of the applicant's visa interview along with instructions for obtaining a medical examination.
Note: It is important to follow instructions from the NVC carefully. Send the NVC documentation (or paying fees) when they were not requested by the NVC will result in a delay in processing.
Source:
http://travel.state.gov/visa/immigrants/types/types_2991.html