@ People who are saying that you shouldn't be monitoring content:
You apparently haven't been a parent, or uncle or person in an authoritative position over a minor. They need monitoring and guidance. In the same manner that you don't let your underage children go to R-Rated movies, go out on unattended dates, drink, etc... you shouldn't let them browse certain websites (porn, social networking sites etc...). What sites you don't want them to be browsing is up to the parent, but its part of being a responsible parent.
If you don't do these things, then you're not being responsible. You are underestimating the amount of trouble and the amount of problems that can arise from young people being unleashed on the internet unmonitored.
@OP: Try netnanny, or you could keylog/trojan her computer.