I'm going to mention a problem about our dads car to my brother and basically tell him in all-caps to not confront our dad and stand down.
How do you think your brother will react to what you tell him?
He will not be able to understand. He does not have a good relationship with our father.
I do not think it's best to tell him about the situation with your dad's car, specially if they do not have a good relationship with each other and they may fight. It's best to just seek help from a professional or just ask for a vague opinions about the car with your brother and not mention your dad at all.
I will tell him to be nice to our dad just because he's our dad.
You should not put your brother in a situation that may trigger a negative response. It's important to remember that your experience with your father is different from his experience and their relationship will be different as well.