A US citizen can only sponsor the following: Spouse, Parent, Unmarried Child Under 21, Unmarried Son/Daughter 21 & over, Married Son/Daughter, and Brother/Sister.
Please note: This information is general in nature and is not intended, nor should it be construed, as legal advice. This post does not create any attorney-client relationship between us. For specific advice about your particular situation, consult an attorney.
Unfortunately, you cannot sponsor your nephew as a family member. As sponsorship is limited to children, parents, spouses and siblings. You may be able to sponsor him as an employee if you have a business and he has the relevant skills or experience.