If it's just for visit, then they need to submit documents which can prove ties to Nepal (meaning they will return to Nepal) financials, lalpurja, business (if any) etc ...I think it's much more difficult to get a visa without a probable cause such as college graduation, pregnancy or illness but you can try it and see what happens
From your side you just need to send your bank statements, account information n recent W2 tax and a letter stating you'll take care of their expenses while they're here in the US