my situation is similar. for the "funds" question, i put the amount of money i had in my bank account plus an amount my parents pledged to give me. then, in my separate letter of explanation, i stated that this money would be going mostly to incidental expenses, and that my spouse and mother-in-law would be paying for rent, groceries etc.
if i had no money saved up at all and was entirely dependent on my spouse and mother-in-law, then i would try to calculate how much it would cost them to keep me fed and sheltered for the duration of the visit, and put that amount. include a letter of explanation regardless. i would not put $0, nor would i put their entire salary, and i think the form will fail to validate if you leave it blank.