Using iTunes match on a computer that has two libraries
In general it's recommended to have one computer account per user (in your case one for your sister and one for yourself), each with its own settings, home folder, iTunes library etc. This also takes care of separating mail, calendars, iTunes Match settings etc. without having to go through a lot of loops. 

In addition you need a separate AppleID for each user (especially if both want to use iTunes Match independently).