you can buy all the software you want, but any educated, computer literate teen can easily find and disable the software. i know there may be passwords and etc, but its probably easier than you think.
what i would suggest is to have the computer in a public area, not in the other room (unless you can work in the other room at the same time) and just simple supervision can deter any teen from doing something they know they shouldnt be doing. for instance, if sister is on myspace, or facebook or something like that, the software will most likely block those sites, although with supervision, i believe them to be ok sites to use. i do believe that even internet browsers, and your antivirus do already have settins to block certain content - so check those out first.
i wouldnt allow her more than 1 or 2 hours a day on the computer, especially in the summer when there is so much you can do outside. when you are spending time together, do things that are outdoors or away from home. when you do allow computer use, see if you can avoid the internet and possibly play a game together or something.
i dont konw how your internet is connected, i know like with dial up you have to have the password to connect, however, with dsl like i have all you have to do is plug it in. if you were to have to be away from home and dont want her on the computer, simply remove a power cord from something - such as the dsl itself, so that there wont be anything that she can do to connect.
i dont want to give you the impression that teens arent to be trusted. shes your sister after all... you should be able to trust her. however, the most innocent sites can bring up the most aweful material. its not necessarily the teen getting into trouble, its the internet itself that can cause a lot of problems.
so basically, constant supervision is the safest bet, that way if something odd does pop up, you will be able to tell if she was looking for it, or if it just happened by accident... you know?
good luck and have fun!