Answered on Jan 31st, 2011 at 12:58 PM
As a U.S. Citizen you can petition your step-mother if the step-parent relationship was created before you turned 18 years old. Since your father and step-mother did not married until after your turned 18, you cannot petition her. Additionally, even if you could petition her, her children would not be included in the petition since there are no derivative beneficiaries in an immediate relative petition. After you father becomes a Legal Permanent Resident he could petition his wife and step-children, but it does not grant them the right to remain in the U.S. after their visitor status expires.