If a person wishes to come to the U.S., he or she must secure a visa from the government, which requires the person to know someone who is already in the country to act as a sponsor.
The U.S. Department of State explains that a person can sponsor family members to come into the country if they are a brother, sister, spouse, parent or child. However, if a person is not a U.S. citizen but has a green card, he or she can only sponsor a child if he or she is not married or a spouse. Do note that family also includes anyone related by legal adoption.
Besides the resident status being an important factor in whether someone can sponsor a family member, the other requirement is that a person must be at least 21 years old.
There are two options for family immigration visas. The family preference visa is available to U.S. citizens and green card holders, but there are a limited number of these visas each year. Using this, a person could possibly sponsor someone with a more distant relationship.
The immediate relative visa does not have limits on the number the government issues. It is for close family members only. A person can only get this type if he or she is a U.S. citizen and for a parent, child or spouse.
U.S. Citizenship and Immigration Services states that another category of individuals who may be able to sponsor family visas are refugees who have entered the country in the last two years. This also includes asylees.