TWD, you don't tell your parents about the webcam for crying out loud.
You set it up, and keep it hidden. Go to the computer in the morning (shower time) and point the camera behind you while your sister's freshening up. Pretend you're doing your homework or something.
Once you've got the 'Film at 11', you can dcc it to me and I'll upload it onto some webspace and let everyone know. We will then be able to give you an honest opinion as to what your sister really looks like to the 'real world'.
Also, if you do this, I'll be your friend.