First of all, your wife, who is your petitioner is obligated to file her own Form I-864 Affidavit of Support, regardless of her lack of earned income. Then, your step father, as the co-sponsor also needs to file his own I-864, with his wife, his "household member" with whom he together files tax returns having to file and sign the Supplement "A" only,i.e. "Contract Between Sponsor And Household Member".
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Yes, She needs to fill out I864 and indicate that she is the sponsor. The dad needs to file his own I864 and indicate that he is a joint sponsor. The mother needs to fill out I864A supplement to the dad's form.
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