You fill out I-864 as the petitioner and your stepfather fills out I-864 as the 1st and only joint sponsor. I-864A is for household members who are related.
This answer is for informational purposes only and should not be construed as legal advice.
Based upon the information you have provided, you should fill out I-864 as the petitioner even though you don't appear to earn enough, and your stepfather should fill out an I-864 as the only joint sponsor.
Cella & Associates has practiced exclusively in the area of immigration law for twenty years. However, the answers provided herein neither constitute legal advice, nor create an attorney/client relationship.