like daka said,
Following are document they would need;
1. Reason for visiting USA (your Graduation Invitation & Letter from University or an International Office)
2. Ticket Booking (Itenary) - dont worry booking doesn't cost money.
3. Income Source: Bank Balance, Their Income Sources (Salary, Investments, Land/Home Rent Income, etc)
4. Property Evaluation by Auditor (Civil Engg. or Architect Consultancy)
5. Intent of Return back to Nepal:
This one is tricky, they should prove their intent of return;
- one way, JOB LEAVE APPROVAL if they (or one of them) are working in any office
- second way, enough property over Nepal which they wouldn't leave it forever
- third way, any other child (for which they need birth certificate of your sibling)
6. Relationship certificate (how your parents are related to you, sounds silly but its required)
7. No Criminal Records (from Nepal Police)
8. Where would they stay; Their Hotel booking or if you have enough space in appartment then your residence CONTRACT along with your residence registration (XEROX)
Above are basic stuff they need to take care before applying for TRAVEL VISA !
I hope it helps