Sounds like they have doubts about the ability for the government to collect from the joint sponsor if needed. I would find a different, well-qualified joint sponsor asap. Submit all the required documentation for the new joint sponsor. Yes, they can and will, without hesitation, deny you if you do not sufficiently answer the RFE. This is not a time to hesitate. You get one shot.
A denial will mean resubmitting a complete new package and a new payment.....hopefully before they an NTA.