How to turn down friend nicely?

Okay so i really wanted to see a movie with my friend today. So I asked her we should see diary of a wimpy kid:Rodrick rules. So she said sure. Well my mom has work so she can’t take me so my friend said that her mom can take us. I was happy. And then she said oh and my brother is coming I asked anyone else and she said her moms friends all in the same vehicle. I don’t want to go anymore because I would feel uncomfortable with people i dont know in the car. How do I tell her?