Can I refuse to give my stepmother citizenship?

You can definitely apply only for your father and not for your stepmother as you are petitioning under the immediate relative category, which does not allow dependents. For you to petition for your stepmother, you would have to file a separate petition. That being said, once your father immigrates to the US, he can file a separate petition under the F-2A category for spouses and unmarried children under the age of 21 of permanent residents and have his wife join him later in this country. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.