There are currently 5 states in US where marijuana is legal and over-the-counter in any licensed store. Also, most states (and countries in Western Europe) have medical marijuana.
Many people I know moved permanently to specifically address this. Even the Catholic and the High Anglican--omg, no pun intended--Church specifically use the term "illegal drug" in their respective codes of conduct; otherwise, they instruct temperance. Practicing Muslims forbid the use of most (if not all) intoxicants, but Khamenei's book on modern Islamic practices permit the use of medical marijuana (Question 1393).
The situation may be more complicated than you think; especially if she has gastrointestinal issues or severe cramping, and she feels entitled to use based on the compassionate-care model.
Depending on her age, she could legitimately study abroad, in say, Colorado, US, and spend as much time and money she wants on it. Statistically, she'll probably grow out of her desire to use, or she mighr pursue medical usage. One need not smoke marijuana for "benefit." There are patches, edibles, inhalers, etc.
However, there is legitimate concern marijuana is a gateway drug to other drugs like cocaine, which are way worse on your body and soul, as well as a culture of people willing to break the law for fun. The risk (in the US) of this has found to be minimal compared to other socioeconomic factors, but doing business with illicit drug dealers is inherently dangerous.
That's the complicated preamble. My point of mentioning all the above is her use may not necessarily be an all-or-nothing phenomenon. The best advice I can offer is talk to your sister about it. Let her know how uncomfortable you feel and that it's selfish to put you in the situation you're in.
In the end, you should honor your parents. If you live in a High Anglican/Catholic/Muslim home, most homes probably, this would be the expectation. Remember, they gave birth to you as well as pay for your Internet access! Secrets are occasionally ok, but on this issue, they are legitimately concerned about her health and want to know what's up. I find it far more likely they would offer forgiveness and act reasonably rather than freak out and create more drama.