Like all parents, they're probably just worried because they noticed changed behavior and are not familiar with the matter (drugs) at all. Family members not (directly) communicating with each other also doesn't help.
If something is on your mind (or your parents for that matter), just confront them in a civilized manner. That's the only way to get it off your (and their) chest. And if it's hard for your parents to take the first step (which it apparently is, judging from the fact they approached your sister first), it's up to you to make the first move.

'm sure they'll appreciate your candidness (and if they don't, there's not much you can do about it). If you did everything in your power to clear things up, it's not your problem anymore.
Being a father myself, I certainly realize that at some point in time, my children will probably experiment with things like sex, alcohol, drugs, smoking, etc. And there's not a damn' thing I can do about it if it is happening outside of my view. Personally, I'm not too concerned this will happen, because I think it's just part of life. To me it's not a matter of 'if', but 'when'.
The only thing I care about is that my children will know they can talk to me about *any* subject if problems arise (addiction, pregnancy, etc. etc.). And I most certainly will (try to) inform them about drugs/alcohol/sex/etc. myself when the time comes to do so. That takes a lot of the awkwardness away.
And I certainly don't think drug tests are an answer. If you (as a parent) need those, you have major trust issues in your home/family, and those (to me anyway) are a bigger issue than a (possible) drug issue and parents should address those trust issues first, because this time the parents think it's drugs, next time it's alcohol, the other time criminal behavior, etc., etc. If the trust is not there, new issues WILL pop up.