[Fey]

Okay, late to the conversation, but I"ll put in my two cents.

Talk to your mother and father together. Inform them that as of right now you're as much responsible for the house as they are, and that you get a say in whom lives there. Your brother is NOT contributing positively and you want him out. Will it SUCK to have to be that person? Yes.

Get your parents to understand that this is for his good, because it is. Kicking him out will show him that bad decisions have consequences. If he persists in sticking around call the cops for trespassing. As his name is not on the mortgage, and at 18 he's a legal adult, he is in fact trespassing if that is how the home owners feel(you and your parents). If, when the cops get there, they discover him in possession of illegal substances you will not be held accountable, nor will your parents.

Do the same if his friends show up, even if your parents don't agree to get him out. His friends have no legal right to be in your house, even if your brother lives there. Use the non-emergency number and call the cops. Tell them that you have a person on premises that has been asked to leave and refuses. The cops will show up and get them out.

Sometimes it sucks to be the voice of what's right, but someone has too.