Can I teach my sister how to drive even though I'm under 21?

I'm 18 and my sister is 17. She currently is learning how to drive so she can get her license. She took her road test once and failed, so she's taking it again soon. My dad drives a Mercedes and i drive a Honda Civic. Since my dad is always busy at work, he never has the time to teach my sister how to drive. Instead, he wants me to sit in the passenger seat while my sister drives. The thing is that you have to be over 21 to do that, correct? I told him that I'm 18 and I'm not going to do it and all he did was yelled at me. He called me selfish and blah blah blah. He said "just drive slow and the police won't get you". He said that I'm selfish and if i don't do it, he's taking my car away. I don't know what to do!

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