Visitor Visa - B2

My Interview is on 9th May, 2014
I am aspriring for the B1/B2 visa to visit my sister. My sister is in US, studing there and she is going to graduate this year, and there are two functions (1 is on june 5th and other one is Aug 9th) which she want us to attend. My parents are not capable to go US as they are well. So my elder sister want me and my younger sister would come and be there at the ceremony.

We,both are going to just visit her, not for staying there. I want to know which document should I show them so that they get convince and give visa for both of us. interview for me and my sister is on same date but diff in timing as we have applied it separately, because younger sis passport got delayed, and in mean time, there was some prob in my DS-160 form, so I missed my appointment on 1st may and rescheduled it to 9th may

My little sis is doing graduation 2nd year and she will return back to continue her studies of 3 years and she will continue her studies in India only. My little sis has applied for visa when she was 15, 3 years back. US counselors has rejected her visa without giving any reason. I would like to know if counselor ask her then what reason should she provide. she is 19 now.

I am pursuing masters her and I am in 2nd year, final year, will be completing it soon. and once my result get out. I am going to join TCS as I have take leave without pay from organization.

My elder sister is also going to come back to India after these ceremony.

Me and my sisters will go alone as she would get free from her exam early. and I will go afterwards.

Any suggestions in interview, in document, if given, I would appreciate if anybody can provide suggestions to get the visa, wether for interview or for document, I would be thankful.