The answer to your question depends on what country your brother or sister is coming from. There are different "lines" with different waiting periods based on how many people apply from those countries each year. Once you petition for an immigrant visa for your sibling and it is approved, you will be given a "priority date". When your priority date comes up, a visa will be available and your sibling will be able to get an immigrant visa to come live in the U.S.
The time period changes often but currently those siblings who are eligible to come to the U.S. NOW, had to wait approximately 16 YEARS if they are from PHILLIPINES, 14 YEARS if they are from MEXICO, AND 9 YEARS if they are from China, India, Dominican Republic and ALL OTHER COUNTRIES.
Note: Only U.S. Citizens can petition for siblings.
The sooner you apply the sooner a visa will be available.
You can check the VISA BULLETIN, to see what the priority date for your country is at http://travel.state.gov/visa/bulletin/bulletin_1360.html
Seattle Immigration Lawyer