1. You would/should not have submitted an I-864 at your interview. That would have been an I-134. For Adjustment of Status, the petitioner will have to submit a current I-864.
2. If the petitioner is not currently qualified, then you will need a well-qualified Joint Sponsor. The joint sponsor will also submit an I-864. If the joint sponsor files a joint tax return with a spouse, then the spouse should submit an I-864a.
Tax returns are not relatively important, although they, among other things, are required to be submitted. CURRENT annual income is king (paystubs, employment letters, etc.). I suggest you and the sponsors become "A students" of the I-864.
Form I-864, Instructions Affidavit of Support Under Section 213 A of the I N A. (uscis.gov)