From everything I've read of her posts, BIL seems to have no real desire to "help" her brother, she seems to really want him to stop because it inconviences her. That is not a criticism, it just seems to be how it is.
BIL, what is it that you want to hear from us? Bear in mind you are talking to a group of addicts in various stages of recovery. If he were living in your house, I'd tell you to kick him to the curb, but it is not your house, it is your parents house, and for whatever reasons, they don't seem to have a problem with it. You do. So if you don't like it, either get your own place or learn how to deal with it.
I personally smoked massive amounts of weed every day for years and I can honestly tell you that the smoke effected nobody but me. Unless you guys share a room, or he is smoking in a common place, you are very much overreacting, which is your right. What is not your right is to dictate what can and can not be done in your parents house. You have stated that you are an adult, time to act like one. Take care.