For B1/B2 visa, no document from US side is needed. If your relatives' visa was approved and you had sent letter, W2 or whatever document from US side, trust me, those did not make any difference in visa decision. Tourist visa is issued only on applicant's merit, his/her ties to the country and reason to return back. US embassy doesn't care if someone has USC/LPR family or friends in US. Your mother, father and brother's application will be evaluated individually and not as a group. I would suggest you not to worry about any documents at this point, just apply and have a good reason why your parents and brother have to return back to Nepal. I see many people (specially Nepalese & Indians) are under impression that invitation letter from their US relative explaining how they want their family members to explore US was the main reason for issuing visa. It was not. If some one has a solid reason to return to Nepal, yes embassy will issue visa.