Yeo... There is no rule on whom you can invite and whom you cannont.. i had couple of my family members fly out here for my graduation.. you just have to send the invitation through the presidents office from your college to them. the people whom you are inviting must present the document as evidence while applying for the visa... they should also include documents which provides reasonable assurance that they will return to Nepal.. my gutt feeling is that.. far "relation" like mama .. maiju may have little hard time.. taking the interview process in consideration.. (you know how poorly those interviewers treats the intervees.. ).. well it does not hurt to try.. Good Luck!!