It's not about the document it's about the strong reason that they will be going back after they get here..... I myself had directly chatted with counselor from visa department..... I found that they mostly look for the reason to be back to Nepal after graduation.....
I hope it helps..... You should show strong reason to go back rather than document..... I had same problem my mom got rejected 3 times.... She never got visa "...... That's I hope the reason in my case..... I m pretty sure it might be for u too
So stop panicking .... Just have a good reason to go back make a reason that ur sister is getting married in magh so they have to go back by that time.... Or sth like that once they got visa it's all gud abt staying for few xtra months