Should I tell her the truth?

My parents have been living with me for 3 months, mom is late stage 6. I’ve been trying for 3 years to convince my father to come live with me, he’s 84, and was not taking care of her well on top of caring for his house and all. It’s been a challenging adjustment but they are very comfortable here physically, but not mentally. She keeps asking when they are leaving, says she’s a prisoner here, the agitation kicks in and she cries etc. I’m not sure what my dad says to her when I’m at work (he’s got a negative attitude and has never acknowledged all that I and my family do for them) but he keeps saying to her they are leaving soon. I just want to tell her the truth, that she’s very advanced in her illness and he just can’t take care of her and their home anymore. Have you experienced saying the truth to a difficult LO?