Currenlty, Au pair Visa is not available for Nepal. The best method would be to invite over your sister to visit you in B-1/B-2 as a tourist visa, that way she will get a 5 years multiple entry visa and the port of entry will give her 6 months permit to stay. for this you or your husband can submit an affidavit of support for financial and other liabilities while she is here. Every 6 months, before her visa expires, she can go to mexico or Canada and renter for another 6 months respectively for the remaining period of her visa. Another option for her, after she is here is to change her status to a F-1 student , that will provide her a valid duration of status, LEGALLY.