Both of my parents live with me because they can't live on their own. I can't work -- can they contribute to expenses without involving tax?

Mom has cancer, Dad has Alzheimer's. Both parents have SS income. My husband works, but until now, we have relied on both our incomes. They are willing to help out--what is the best way to go about this? Charge rent? Have them hire me as a caregiver? (taxes involved here) Just have them pay some of the bills?