It definitely works. Wedid the same
there are other threads like that. Of course they would. I answered a member with the same question.
My husband and mom in law (both USC) filled the affidavit(bothe sponsored me). They just added their income on the I-864, and his mom separetly filled I-864 A "Contract between sponsor and household member" . --864A works if they form a household for the past 6 months.
If they dont live together, she can sign separetely I-864 as a joint sponsor .
The 2 options to include the mom-in-law are either as a Household member or a joint sponsor depending on whether they live together or not.
Otherwise adding the incomes definitely works and dont doubt
Good luck