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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Who is the immigrant? You may want to consult an immigration attorney if you do not clearly meet the income guidelines- the 864's are tricky.
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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