I find that often my child when tattling has something that she did that I did not find out about. I simply acknowlegde the info she gives me about another , and then swiftly ask her what she has not told me, about something she has done, that she thought I should not know.
Very quickly she will offer up some act she is not proud off, don't get mad or discipline, simply thank her for trusting you enough to tell you.
Soon she will only bring the information you really should now about. :)
Another trick is to have the child suggest what you should do about it, and then encourage her to use that solution herself next time. Empower her to handle her siblings, friends, parents , teachers, neighbors ect .
example. she comes in and tells you that her brother just finished all of the popcorn and did not share the last of it. " have you ever done something like that ? when was it ? where was it? What could you have done instead? " Thank you for telling me,now next time your brother does something like that what could you do to encourage him to share? Great ! do That. :)
This really works with my brood of five.