Should I be concerned by my sister driving barefoot or in flip flop?

My little sister just got her driving licence and she always drives the car without shoes or with flip flop. I told her it may be dangerous but she doesn't care because she doesn't want to wear "real" shoes.

I tried to convince her to leave an old sneaker on the car to at least cover her right foot but she did in once to please me and never wore it again.

Should I be concerned by my sister driving barefoot or in flip flop?
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