Visitor Visa for unmarried cousin sister with my Parents (already got Visa)

[knuguess]

My parents are visiting me for my graduation ceremony next month. They already go their visa approved. My cousin sister (Mom's sister's daughter) is presently not working and has free time. So she also wants to tour the US. Her interview is scheduled April 17. I've been reading forums and found its near to impossible for young single unmarried to get visitor visa. I did not knew this before applying. 

She finished her studies in 2011. She has been volunteering for a non profit organization from 2012 till now. She got paid small amount for the work until Dec 2014 but not now. So we did not mention any salary or previous employment in DS-160 but mentioned Not Employed - Volunteering for <Organization_Name>. Her parents are both working in State Government and a Bank. They have good salary. They also have good amount of property and cash. Her mom is the sponsor in DS-160. Also, we mentioned travel companions as my parents names in DS-160.

I'm going to send her an invitation letter from myself, graduation ceremony invitation letter from my school. She will be preparing a property statement of her parents, property statement of herself, bank balance statement of her parents, herself. She will also carry pay-slips of both her parents for last month, visa copies of my parents (her travel companions).

Now the questions
1. Should she carry any affidavit for support from her mother ?
2. During the interview, what should she project to get the visa ?
3. Is there anything we can do to increase her chances of getting tourist visa ?

Appreciate any help.

[ashuneel]

1.Yes affidavit should help.

2. Tell the truth. attending graduation and visiting USA.

3.tell the truth.