Cost-U-Less Company

I have received my sister's car insurance bill for YEARS from them at my parent's residence. Even though we've been estranged from her for years as well. As their caretaker these last couple of years in their declining health, I've put Return to Sender on the bill endless times to no avail..
When I've called the company to let them know, they can do nothing because I'm not the insured. I've said they could Call their insured and get the correct address. Especially if they get in an accident, they said they couldn't talk to me about anything they could do. And did nothing.
This put undo stress on my parents for the remainder of their years.
They're gone. And here I write Return to Sender again. And.. Not At This Address again. While the place is for sale. Wondering if the new owners will have to endure this nonsense for the next several years.
I wouldn't use you if your service was free. Because it's already shown to barely exist.