I'm sorry, I would just like to make another plea for you not to call the police. Jail is a really messed up place and if your sister was sent there, she would probably never be the same again and never forgive you. Believe it or not I'm in a very similar situation, but nothing dealing with drugs. I'm going to college too, I'm 21 and I have a younger brother whose 20 and due to an internship I have right now, I'm living at home again. We don't get along at all and infact fight quite frequently. Not only do we fight but he steals my stuff while I'm gone at work and has no respect for anything I own. Sometimes I think I would love for my parents to kick him out of the house just because he's such a jerk to everyone, but they won't. I understand why, they love him and he obviously isn't mature enough for the real world yet, so they'd rather put up with the crap, then worry about him on the streets. Like I said you're 22 and in college, you're going places. Maybe your sister will straighten out as well, she just needs some time to be crazy, and your parents are giving her that by letting her stay at home rather than forcing her to sink or swim by kicking her out. Don't ruin her life and your relationship with your family because your 20 year old sister is smoking weed(she's not even close to alone in this). Other than it being illegal, there's nothing wrong with it. If you need to talk to someone or tell someone, talk to me, I'll give you my number and we can chat. Anything but calling the cops.
[EDIT]I guess I should tell you that I am biased because I do smoke weed. [/EDIT]