My threat would be dad becoming their driving teacher. Dad’s patience and hormonal teenage girls don’t mix well for driving lessons.
My father was going to teach my oldest sister how to drive. They had one "lesson". She ended up in tears and vowed to never, EVER drive again. Matter of fact, she didn't get her license until she was in her 20s.
He never even attempted it with middle sister or me.
First time I was ever behind the wheel of a car was in driver's ed. I hadn't even driven in a simulator yet. Teacher chose me to drive first (that was his FIRST mistake). I had to back out of a parking space in a parking lot full of teenagers and teen drivers, then drive down the hill behind our high school. I didn't hit anyone on the way out of the space or the lot, so yay me? However, having me drive down the hill was his second mistake. The sidewalk went at an incline like the road. The houses all had retaining walls so that their yards were level, creating a ramp effect. Three guesses where my right wheels went. Teacher tried to brake, but the incline and the car tilting made his foot slip off his brake (third mistake). I managed to hit my brake, and he was hanging over me, screaming at me while I was dissolving into hysteria. Fourth mistake, as I was so scared I couldn't do anything about our situation. I didn't want to touch anything, and the two boys in the back seat were hollering too. He managed to reach over & put the car in park, then shut off the engine. Then we all calmed down. Well, most of us did.
I have NO idea why they let me behind the wheel again, or why I agreed to keep trying. But I still remember that damned retaining wall!