On the interview, would the immigrant ever be asked what their relationship is with the joint-sponsor?? Also, would they ask questions regarding HOW they support you?
I believe that's irrelevant to the process, USCIS cares about the fact that someone is on the hook for you till you are a US citizen. Unless the person who is a sponsor is using illegal means to obtain $$ that is in turn send to you, you have no need to worry. The joint sponsor could be sending you a bag of kosher meat every Thursday, 20lbs of potatoes, rice and kosher beets, could be considered supporting you. I don't think USCIS expects them to send you cash on a monthly basis.