Whether you're employed or not is not relevant. It seems like you have two options:
(1) your wife chooses to use your in-law's income/assets for qualifying for the povery level guidelines, then no joint sponsor is required.
(2) you use a joint sponsor, who needs to have enough income, it doesn't matter what your wife's income is because it's presumably below the qualifying income anyway, otherwise no joint sponsor is necessary. Including household member's income is always optional, so it's not required, and wouldn't help if it is still below the qualifying income. If you look at I-864, your household size is basically determined by whose income you choose to include, regardless of the actual number of people living with you.
