They may if they have all the proof to show the relationship was established before your 16th birthday; but you will have to wait many years before there's a visa available for you.
They can sponsor you for a green card if they are at least 21 years of age. However, depending on your country of birth, the process can take betwee 11 and 23 years.
I agree. You may want to explore other avenues for immigration given the lengthy wait times for siblings of a U.S. citizen.
Lynne R. Feldman, Attorney at Law
Concentrating in Immigration and Nationality Law
2221 Camino Del Rio South, Suite 201
San Diego, CA 92108
phone: (619) 299-9600, facsimile: (619) 923-3277
Formerly Adjunct Professor -- Immigration law
University of Illinois College of Law