Are they on drugs? what do we do?

They obviously are. You may want to talk to them about it. It's your house after all. Don't expect a positive attitude though. But they certainly are not free to act as they wish in your house specially that your dad doesn't like it. Some it comes to either you've got to kick them out -- you obviously don't want to do that -- or you have to tolerate their behaviour as long as they don't hurt you or yours. Also, smoking pot does not cause the parents to be abusive toward the children, but there is also the pills. I guess you should watch their behaviour with their children. If they are abusive, you could report them to Child Protective Services. This however will not establish family peace. It's your choice what you decide to do though, and no one should blame you for your decisions.