It's the konami code, up, up, down, down, left, right, left, right b, a, using a standard keyboard's arrow keys and the 'b' and 'a' keys. You'll need to put it in in-game while the sister is sitting around the room. You need at least version 1.5.4. Notice, that WolfRPG Maker uses the key's 'inner' codes, rather than simply whatever 'left' 'right', etc is, so while a majority of keyboards should work just fine, there is a possibility that alternate keyboards, such as kanji, or different layouts, could be different keys. If you have an alternate keyboard setup, try typing the same keyboard key as where a qwert keyboard would have the 'b' and 'a' keys.
^ ^ \/ \/ < > < > b a