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!

And you live where?

Of course it depends on the laws in the state in which you live. Eg in the UK, she can start driving age 17 if she has her provisional licence, and be supervised by someone age 21 that has held a licence for 3 years.Usually in the UK at this age, she would have driving lessons from a registered instructor.
So don't let your father bully you into breaking the law.
Something you could do to help your sister- find an off-road site like a quiet car park or field, patiently help her practice use of gears, parking, reversing etc.

I would talk to the dmv about it and they'll let you know what age you can teach your sister, maybe you can ask the dmv to tell the age to your parents so they don't bother you about it

Go to a field, a long driveway, anywhere that is not a road or a park & drive. Somewhere, where the police don't monitor, then let her practice. If you have a friend that has a giant driveway, then use it. You might even ask if a high school teacher or a school staff member would like to do this for your sister, and explain why.

Your father was very wrong to tell you to break the law so he wouldn't have to take care of his responsibilities.

Chong

Depends on the state. Check your state's driver's manual. In the section about rules for learner's permits, it will say how old the person in the passenger seat has to be. If you're under that age, it is ILLEGAL for you to be the one in the passenger seat when someone is learning to drive. I don't know exactly what will happen if you do it anyway, but unlike your dad taking away your car, it would probably go on your permanent record. Maybe somebody at the DMV can give you more details. Explain to your dad that it is actually illegal for you to teach your sister to drive (assuming that you find that you are indeed too young). There's nothing selfish about obeying the law. It's selfish of him to take away your car because you're not willing to break the law for him. Maybe you can find someone else to teach your sister how to drive. There aren't any restrictions except they have to be at least a certain age and they obviously have to have a driver's license. A friend would be fine as long as they're trustworthy.
Also, do you own your car, or does your dad own it and allow you to use it? If you own it, he can't take it away.

Where do you live

Find her a driving teacher, I suppose that may help.

No. You can't do that. That is illegal.

No