America Supports dual citizenship but nepal doesnt. So, the problem might rise from nepal's side. Also, about the green card sponsorship, if you are under 21, its fast.If you are above 21 and unmarried, its slow. IF you are above 21 and unmarried, you might not be qualified. Anyway, check through USCIS. They explain everything. Malai padhnu alchi laagyo. ANyway here is the link.


http://www.uscis.gov/green-card/green-card-through-family/green-card-family-member-permanent-resident