It seems that you want to know if you need to be at the Slingbox location for the first time setup.
As a matter of fact, it is not necessary for you to be at the Slingbox location. Your sister needs to configure the Slingbox for Internet Viewing in order for you to get the remote connection.
Once she sets it up, you should be able to get the remote connection to the Slingbox from your remote location. Just make sure to check the tech specs on both of the locations.