As another big brother, I can relate.
It's just weed, it can even be healthy for her social life. However, you have a strict responsibilty to explain her the possible dangers, you have to forbid her from getting high around men she doesn't know and trust, and you have to do it the most dominant way possible. I've seen too much shit, I know how ovulating girls react to weed and how they feel afterwards, you don't want your little girl to do something she will be extremely ashamed of later. They have to do mistakes, but not that kind of mistakes.
Also, you have to remind her that while weed and acid are usually harmless party drugs as long as she doesn't get attached too much, she must stay away from the hardcore ones.