You should give your car sometimes for short periods like 2-3 hours or so ... long periods just refuse and say you need the car ... you can ask when you will get the car back ... if it's more than 2-3 hours then tell them you need to go to a friends place or to the workshop and need the car, or even a job interview ... do this everytime they ask.
when they take the car and are late in bringing back the car then start calling them every 10 mins saying i need my car ... once they come back grab the keys and drive off to sea view and have an ice-cream or something.
when you keep calling them they'll get uncomfortable and will prefer not to take your car coz you bug them during their peaceful cruise or shopping or whatever they are doing.
at the same time you can be polite and respectful to your elders. after a while they'll get the point you're trying to make.