Really do need to know a little more. Most chat programs have some sort of chat log ability. But just on a quick search of download.com i found this
Actual Keylogger - Free software downloads and reviews - CNET Download.com Never used it Dont know how well it works but its free and download.com seems to be a fairly safe site.
that being said... If its a teenage sibling i say give them the privacy they want. let them download the porn or whatever. If they are really getting into serious trouble.. IE hacking or something destructive a keylogger might not be the best answer.
Keep in mind most kids that are over protected and kept on a short leash turn out to be some of the wildest adults. Ever met a preachers daughter? They always know how to have a good time or how to cause trouble.