Can I apply for a greencard for my brother or sister?

The answer is yes if you are at least 21 years old and a US citizen. US permanent residents are not eligible to apply for their siblings. Since siblings are deemed eligible for an immediate visa, they must wait after the USCIS approves their petitions for the priority date of their application to be reached by the National Visa Center until an interview will be scheduled for them in their home country. This could in many instances take several years.