Be careful then - her actions can affect you if you live there. I saw kid get blamed in court for his brothers possession charge. Judge didnt know what was true so she put him in drug court just to keep an eye on him. He went through 6 months of drug court becuase of it and it wasnt even his. never once tested positive or got in any trouble but he shouldnt have been there to start with. I've know this kid a long time - he's no angel but it was unfair - i was so proud of him when he prooved everyone wrong.
You're parents ignoring the risk she is placing on your life is irresponsible denial. One of the main reasons i've been so hard on the consequences with my son is becuase i know the effect it has on his sister. Get yourself some help - maybe through alanon you can learn tools not only deal with your sisters usage but your own parents denial.