What makes you think she already hasn't made this obvious suggestion? It's complicated as follows:
She: "Why don't you rent a car to spare mom and dad from all the driving?"
He: "It's a hassle and then it's expensive. I see no point."
She: "The point is that mom and dad are in their 80s and it's risky for them to do all that driving when you can easily rent a car."
He: "There's no such thing as an easy car rental. I'll end up standing in line for an hour and then it takes another 45 minutes just to go through the process."
She: "You're being so selfish, making mom and dad at their age pick you up. Why don't you GROW UP, act like a man and rent a damn car?"
He: "Mind your own bleeping business, Sis."

Hope this clears it up for you.