Principal applicant's spouse and unmarried sons and daughters of any age, or close relatives by blood, marriage, or adoption who are not members of some other household and who will reside regularly in the household of the principal applicant, are defined as immediate family members. The sending government should recognize the close relatives as dependents.
No, your nephew or niece should seek to obtain the appropriate visa for the purpose of travel.
Yes. For example, a recently widowed, divorced, or aging parent may have closed a former household with the intention of becoming part of your household. You must be able to show that he/she is financially responsible for the new family member.