my experience in my case
don't worry
first of all
your sister as the petitioner received the affidavit fee bil for $65, as soon as she paid it, NVC will send the affidavit form to the petitioner (your sister) and she has to return it w/3 last years tax returns, W2s, and employment letter.
In the meantime your mother, at her country of origin will receive the form DS3032, to declare if she wants an agent (lawyer, or your sister) to receive her correspondence at one US address, or she wants to receive it at her address in India.
As soon she send this form, NVC prepare the IVDS230 (Immigrant Visa fee bill) for $335 to the agent (could be the agent she choose in form DS3032 or your mother directly if she decided to received NVC letters directly).
As soon as the agent pay $335 and NVC receive it, NVC prepare form DS230 part 1 and send it to the agent.
When NVC receive form DS230 from the agent, and affidavit od support form from the petitioner, the begin the review process, if it is complete the finish the review and send the case to the consulate , if not they ask them for more info and review again and finally send the case to consulate.
that was my experience
but you can check this out at nvc website:
http://www.travel.state.gov/nvc.html
When an applicant's priority date is close to becoming current, NVC will send an Affidavit of Support (I-864 form) processing fee bill to the petitioner and an Agent of Choice and Address (DS-3032) form to the applicant. Once the I-864 processing fee is paid, NVC will send the I-864 forms and instructions to the petitioner. Once NVC receives the form DS-3032 from the applicant, NVC will mail the Immigrant Visa (IV) fee bill to the agent of choice. Once the IV fee bill is paid, NVC will send the Instruction Packet of forms and information to the agent. The exact contents of the Instruction Packet will depend on where the applicant will be interviewed for a visa. For instance, the Instruction Packet we send to people applying in China is different from the one we send to applicants in France. Information in the Instruction Packet that can currently be downloaded from this web site is listed below: