How should a 40 year old Step-daughter with a 2 yr old child pay for room & board when she has little money?

Because of a divorce, step daughter and granddaughter moved in with no place else to live. She is working and making some money, but says she needs it to pay for credit cards and bills. How do you suggest that she pay her way for living with us without paying any money for room and board. She buys own food and pays a sitter for the child? How long should she be allowed to stay with these conditions and how can I motivate her to move someplace else after 6 months as it is disrupting our home for me and my husband?