You will find those answers in the "instructions", but....You should know that the sister's current income will come from her most recent tax return. Essentially, her income is the total income minus her husband's gross income. Yes, it is best practice for the joint filing spouse to provide an I-864a whether they have income or not and whether it is needed or not. His current income will come from his pay stub, but hers from the tax return. Your petitioner should submit an updated/corrected affidavit as well.