Brayden is right if you are using 2007. It's a lot crazier if you are using PowerPoint 2003/XP.
Here is what I found in the help file...
~~~~~Change the default design for new presentations:
When you click New on the Standard toolbar, Microsoft PowerPoint displays a new slide that uses a default ("blank") design. You can change this design so that it always includes certain elements, such as a logo or a type of background, font, or bullet.
On the Standard toolbar, click New .
On the View menu, point to Master, and then click Slide Master.
On the slide master, make the changes that you want to be reflected in every new presentation.
On the Slide Master View toolbar, click Close Master View.
On the File menu, click Save As.
Note In order for blank.pot to become the default, it must be saved to the default path in the Save As dialog box.
In the Save as type box, select Design Template; in the File Name box, type blank, and then click Save.
Close the template.
Note You must name the template "blank.pot" regardless of what language version of PowerPoint you have and which language is installed. Also, the variation "blank presentation.pot" will not work. ~~~~~
So it appears that your sister somehow saved over the "blank.pot" Based on the above, I think all you have to do is open a new file, find the blank layout under the slide designs that are available to use, apply it and save it per the instructions above. I don't think you need to recreate the wheel by going into the master, etc. In the future, your sister can save her templates, but they need to have their own name.