https://www.uscis.gov/sites/default/files/document/forms/i-864instr.pdf
page 10 of I 864 instrctions
Part 7. If your total household income does not meet the requirement, you may submit evidence of the value of your assets, the sponsored immigrant’s assets.
However, if you are a U.S. citizen and you are sponsoring your spouse or child age 18 years of age or older, the total value of your assets must only be equal to at least three times the difference
Sincerely, i would still have the joint sponsor
Very sorry for your father's illness/ may he be on the road to recovery