Visitor Visa help for my mother and sister

[Babubhai]

I am inviting my mother and sister for the first time to US.

Do I need to send two separate sets of documents for each of them, even though they are going together for the interview?
Do I need to send separate invitation letters to each and separate letters to the consulate for each? Will a combined letter for both do?

thanks in advance

[immihelp]

You can make one set of documents, except for I-134 which has to be one per person.

One invitation letter is ok.