Oh dear
. Can we assume from what you've posted that she is is taking her A level exams this summer? If that is the case, then grounding her until they are finished would make sense - smoking dope is going to affect her short-term memory and will not help her grades.
But in the longer term, you'll have to accept that she is legally an adult, and if she chooses to indulge in an activity which is illegal, she will have to run the risks and bear the consequences of getting caught. As long as she lives under your roof then you can apply your rules, but once she's at university then there is nothing you can do about it.
You say your DD continues to get excellent grades and nothing in her previous behaviour led you to believe this was going on. Your DD1 presumably graduated and is now a functional working adult, and you never had reason to suspect she was smoking pot either. Without wishing to downplay the health risks for susceptible individuals, given the length of time she and her sister have been indulging without any apparent ill effects, I doubt if she'll listen to what she will perceive as scare stories.
It looks as if damage limitation is going to be your best bet. You need to talk to your DD about the potential damage to career prospects that a criminal record can do. A huge number of jobs require a CRB check nowadays and a fair number carry out random drug test in the workplace too. Tell her that smoking dope means you can't drive a car - cannabis shows up on tests for FAR longer than alcohol does, and that in spite of what people might tell her, it DOES affect your driving ability.
And above all, tell her that smoking dope regularly will almost certainly affect her exam results at university. All that money potentially wasted on a poor class of degree could be a lifelong source of regret. It is also an expensive and smelly habit, and one which might limit her social circle to those who indulge, or are prepared to tolerate it.
This might not seem a big deal now, but it sure as hell will be in a few years time. Does she really want to be part of a semi-criminal sub-culture within her own professional circle? Teachers, doctors, lawyers who smoke dope? I've met 'em all People who use drugs, even just weekend pot-smokers, tend to stick together because they have a little "secret" habit (which most of their colleagues have guessed anyway
.) And druggies are tediously dull company.