Yes, your U.S. citizen brother or sister can sponsor you for a family-based green card even if you are over 21 and married. They must file Form I-130 (Petition for Alien Relative) on your behalf. However, as a married sibling of a U.S. citizen, you fall into the F4 visa category (Fourth Preference Family-Sponsored Visa), which typically has a long waiting period due to annual visa caps. Processing times can vary significantly based on your country of origin. Once your priority date becomes current, you and your immediate family (spouse and children under 21) can apply for immigrant visas or adjustment of status.