I can imagine you are wondering if anything like this has happened to them when in your stepbrother's care. Your other responsibility is to your kids. Other discussions about this can take place between you and your step-brother but not the first one and only with your dad and step-mom's permission. I can imagine you feel responsible because they were in your car, but you are not his parent. Let him in turn tell your stepmom and they can proceed from there. Perhaps tell your dad first since your relationship with him is better. They in turn can discuss this with the parents of the other boy. This needs to be handled by his parents directly.
Dont forget that these 2 boys are minors and you are not your step-brother's guardian. Of course you need to talk to your dad and step-mom before your step-brother.
