Family-based immigration begins with a petition filed by a qualifying relative. The following chart indicates which relatives may petition. No other familial relationship allows for a U.S. citizen or LPR to petition for a relative. Note that the list is different depending upon whether the petitioner is a U.S. citizen or Lawful Permanent Resident.
Petitioner is a U.S. Citizen Petitioner is a Lawful Permanent Resident
Parents (US Citizen child must be 21 or older)
Children of any age, whether married or unmarried (includes children adopted by the age of 16 and step-children acquired by the age of 18
Unmarried children of any age
Siblings (including half-siblings)
Although all of the above relationships qualify, they are broken into several preference categories and immediate relatives. The category determines how long the alien relative must wait for a visa number after the underlying petition is approved. Only immediate relatives are eligible to receive an immigrant visa without having to wait for an available number.
To qualify as an immediate relative, you must be the parent, spouse, or unmarried child under the age of 21, of a U.S. citizen. For immigration purposes, there is no such thing as an "immediate relative" of an LPR.