In this particular case, sister site means nothing more than having more than one forum that you are the 'main admin'.
PB forum owners: If you have more than one 'main' forum, ask in Affiliates but if you have sister sites, forums that are either used for practice or are linked to the other forums, this is the place to be. Share your 88x31 banner or write out a description of that/those forums. Include links to that/those forums. A picture/full banner is optional. Don't forget to include the name of your forum.