Is this rude of me?

My MIL invited my husband, my son and I for dinner tonight at her new place. She lives 1 1/2 hours away from us. We said we would come. Then my husband asked us if we could bring his sister and her son with us (she doesn’t have a license or a car). Would it be rude of me to say that she has to give me some money for gas? I literally drive her places all the time and she never give me any gas money. I don’t want to be rude and say she can’t come with us if she doesn’t help with the cost, but I’m tired of being the free taxi service and I don’t think it’s fair that I have to drive her places on my own dime frequently. What do you think?