You can't stop your sons from smoking marijuana. You can keep it out of your home. Unfortunately, this is becoming a common problem for parents. The eighteen-year-old doesn't need to bring that into your house. Hopefully he has an exit plan in place. If not, it's time to have the pay rent or move out talk. No one should be smoking weed in front of your younger kids. My advice, be mean. Let your eighteen-year-old know that you are not going to have marijuana in your home. Period. Legal or not, it's your house. At least that way the eighteen-year-old will have to go somewhere else to do it, not in front of your younger kids. Also, having an older sibling do drugs is not a predictor of behavior. Your children are watching you. Be the example and keep the weed out of your house. As for fixing the kids who are smoking, if they are not in your home and they support themselves, in some states, it's legal to smoke Marijuana so as adults there isn't anything you can do. I wouldn't blame yourself for being a good parent. Doing drugs is on them. They may stop doing it after a while. Sometimes it takes a good influence outside of the home to end these behaviors.