How do you deal with tattle taling?

My youngest daughter has made a habit of tattling on her big sister. On the one hand, I want her to tell me when something is wrong. And, I want her to feel comfortable telling me everything. On the other hand, I don't want her to tattle on her sister, or anyone, fifty times a day. So, this is a bit of a pickle.

I've been telling her to stop tattling on her sister. But, she cries so sadly whenever I do so. Which makes me feel bad, on top of not knowing whether or not I'm taking the right approach on this particular issue.

This, in my opinion, is the most difficult aspect of parenting, figuring out what is and is not the right thing to do for ones child.