So the question is whether your son should act as your "household member" or as a "joint sponsor". The I-864 allows a sponsor to count a non-dependent relative living in the same household in the household size in Part 5 #7, but the instructions say that "Such relatives may include your mother, father, sister, brother or adult children, if they are living in your residence." So you can clearly count your son there if you want, but it's not clear whether you can count your daughter-in-law there.
So here's how it would work in each case:
If your son acts as your "household member": You will fill out an I-864 as the petitioner. Your individual income (Part 6 #7) will be 0. Your adult son, who you've chosen to include as part of your household size in Part 5 #7, will have his info and income listed in "Person 1" (Part 6 #8-10). Your income and his income will add to your household income (Part 6 #20), and this needs to be sufficient. Your son will fill out an I-864A as your household member.
If your son acts as a "joint sponsor": You will fill out an I-864 as the petitioner, with your individual income and household income both being 0. Your son will fill out a second I-864 as the "only joint sponsor". He will fill his individual income in Part 6 #7. If his own income is not enough, and he needs to count his wife's income, then his wife's info and income will be listed in "Person 1" (Part 6 #8-10) to sum with his income to add to his household income. In that case his wife would need to fill out an I-864A. If he doesn't need to count his wife's income, then his wife doesn't need to fill out an I-864A, and "Person 1" will be left blank.
