why do we use cô but not chú? Or why do we use anh but not chị?
Well in this case you can infer that the lady is older than the male. The male is slightly older than you but the lady is perhaps a generation older. There is a nuance to it.
i think they're asking in regards to duolingo; it's always "cô ấy" and "anh ấy", and never "chị ấy" or "chú ấy"
"She cooks food and he sleeps." should be accepted