You are using Dial up? If yes, you may struggle to have both LW's running at same time.

Try this: Go to LW's menu bar, Tools -> Options -> Advanced -> SuperReallyAdvanced -> Firewalls. Make sure both your connection & listening ports are different to the ones your sister is using. Change manually if necessary. Choose a port number between 49152 - 65535 ... sample image on this setting window can be seen here: Manual port forward instructions & sample image.

It might also be a good idea to get a copy of the Gnutella.net file your sister is using & drop it into your LW preferences folder replacing your older one ... but LW must be closed whilst you do that.

Your preferences folder will be found at either C:/Documents And Settings/your_username/.limewire
or
C:\Documents and Settings\*yourusername*\Application Data\limewire
The latter location you can only find if you go to View options & enable Show Hidden Files & Folders. Application Data is a hidden folder.