For tax purposes, since you filed joint returns with your husband you are automatically a household member. One difference between the main I864 and I864A is that a household member (for I-864A) doesn't necessarily have to be USC or LPR; one just has to be a household member. So yes, you'll have to provide a copy of form I-864A if you want husband's parents to get their visas. Haha, you realized you suddenly have some power to determine your husband's parents faith. Welcome to the some of the nuisances of the behemoth called USC immigration.
BTW, both the sponsor(your husband) and you will have sign the I-864A.
NB: I'll ask admins to move your question to a thread on its own.