Family unification is the most important goal of modern United States immigration policy. The clear majority of immigrants granted admission each year enter on family-based visas. These visas permit specific close family members of United States citizens and LPRs, such as parents, spouses, and unmarried children under 21, to gain admission into the United States.

I. Eligible and Ineligible Family Members

Yet some types of relatives aren’t eligible for family-based immigrant visas....