Saathiharu,
  I need some information. I do have a US citizenship and my mom is in process of getting her GC. We are planning to apply for my sister. Does anyone have any information whether or not you can sponsor for more than one relative at a time? I could not find anything. 

And does anyone have an experience on a realistic processing time for siblings nowadays?

I would appreciate your thoughts.

You can apply as many as you want as it is based on queue system. Your parents will probably get it in 1 year or less than 2 years. As for your sister depends on her age and marriage status. if above 21, might take 10 years. So you should apply ASAP. 

Thanks, Freedom2012. My only thought process has to do with the credibility of the sponsorer if you apply for too many. Since the sponsorship is directly related to one's financial ability, wouldn't that lead to a limit? 

Do you or someone else here got some reference material on this?

 @eutaB4: you have said it right. It also depends on the financial ability that determines the number of family members you can sponsor at a time. Technically you can sponsor as many family members as you wish (spouse, parents, siblings), however, you have to show affidavit of support/financial proof that you would be able to support them once they are here by showing your annual income or total household income, bank balance, real estate if you have, other assets including stock investments etc (or they can show their properties/bank balance in back home as proof that they can support themselves). 

In the form affidavit of support, there is a poverty guideline that determines how much minimum you should earn at least to support to how many number of people. This is a very simple and easy to follow guideline and determines the number of people you can sponsor based on your annual income, bank balance and other assets. 

Hope this helps! Good Luck!

The affidavit of support i think comes later when the GC application has been approved. It is not part of the initial process. It is a simple form and by signing it you are just saying you wont let someone be a burden to the government for 5 years. I know many people who came here and the petitioner is not rich. I dont think they look very strictly at the amount of money you have. I do not know anyone whose visa got rejected that way. As long as you sign that form, it should be more important than the money. You can easily bring 2-3 relatives here on a modest income. The biggest headache is the waiting game. Except for your parents, siblings will take a long time. If i were you, i had file asap to get the priority date.  

I cant seem to put a link here but google poverty guidelines uscis and download the link.

Affidavit of support only needed once application is processed and just prior to interview which means even if you apply together now you will have to do all the affidavits together. And as mentioned earlier brother and sisters will take more than 10 years and you should go ahead and apply soon as I think family visas for brothers and sisters are goin to go i think if they come up with comprehensive immigration reform . It was part of prior bills as they proposed moving towards point system like in Canada .

@supersixnine: Very thoughtful points. Noted. Thanks.
@Freedom2012: Thanks. I found out the form that you mentioned.
So interpreting your threads, you should be able to sponsor as many people as your minimum 125% HHS covers for your current family requirement plus the sponserees, correct? And this perhaps should not imede the speed of processing?

I seem to recall that I have seen a (USCIS) page that shows an average processing time for immigrant visa for different countries. Have any of you ran into that? 

National visa bulletin
Can just google it
Published every month will give you a good idea