Kavitha,
Documents that my sister took with her:
From me:
Invitation letter
Letter for Consulate
Affidavit of Support
Last 2 pay stubs
2 bank statements
Letter from Bank
Employment letter
Copy of passport pages
Copy of I-797s and I-94s
Copy of lease agreement
Copy of Drivers License
From herself:
Her photos with me
Her marriage certificate
Her Son's Birth certificate & School letter
Photos with her husband and son
Affidavits from her husband and our parents
Copy of her bank passbook
Car ownership papers
Her husband's Employment letter
IT may be different for your sister since she is still a student and presumably not married yet.
They asked only those questions which were related to her strong ties with India. Her interview went like this.
Q: Why do you want to go to US
A: To meet my brother and to see places
Q: What does your brother do
A: He is a Software engineer with <abc> company
Q: Do you have any child
A: Yes, I have a 8 year old son
Q: Where will he stay while you are in US
A: He'll stay with his maternal grand parents and his father
Q:What does your husband do
A: He is a doctor with state government
Q:Can you show me your husband's employment letter
A: < showed the letter>
Q: Can you show me your brother's employment letter
A:<started pulling out the letter> but the officer asked her to handover the entire packet to him.
He just browsed through all the documents and said
"You are granted Visa. Enjoy !!" and gave back everything.
the idea is to prove that you will come back for sure. If you have any other younger brothers or sisters in India, it might help.
She should also take a letter from her college that she is a full time student. It would also help if your parents have some property which they can transfer in her name. My Brother-in-law transferred his car to my sister's name and started building her bank balance 6 months before the interview.
Never mention pregnancy as a reason to visit US.
another idea could be to convince her principal and make up some story that her US visit(to some specific place) would help her studies or college in some way, or something like that.. Just a wild idea!!!
Good luck !!