There is a 3-step process for a brother or sister to become a legal immigrant:
The USCIS must approve an immigrant visa petition that was filed on be brother or sister.
The State Department visa bulletin must show that a sibling immigrant visa is available to the sibling, based on the date that the immigrant visa application was filed.
If the brother or sister is outside the United States when an immigrant visa number becomes available, the brother or sister will be notified to go to the local U.S. consulate to complete the processing for an immigrant visa. If the brother or sister is legally inside the U.S. when an immigrant visa number becomes available, he or she may apply to adjust status to that of a lawful permanent resident.
A sibling is a brother, sister, stepbrother, stepsister, or adopted brother or sister. For the necessary sibling relationship to exist, each person must have been a child of at least one of the same parents. The siblings need not share the same biological parents as long as both became “children” at the appropriate time (before the age of 16 in cases of adoption, and before the age of 18 for stepchildren).