Is it fine for my sister (13, F) to be sitting on my friend’s laps (15-17)?

I am a 16, M and we all drive to school together.. we do squeeze in a bit and she gets the most squished so she will put one of her legs on me or on my girlfriend. My friends have offered her to put her leg on them or sit on their lap and she sometimes does do it. They make sure she is also buckled as well.

Is it fine for my sister (13, F) to be sitting on my friend’s laps (15-17)?
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