I would highly recommend you do the armor in foam.
It's fast, cheap, and can look fantastic.
If you decide to go that route, I would recommend one of two methods:
1. Use Pep armor to create templates
2. Make a duct tape mannequin and create the parts exactly to fit your sister.
I recommend #2. Here's why:
The Pep armor has to be perfectly scaled. If it's not, you'll have to start over, or your sister will have awkwardly-fitting armor. After all this IS for a girl. The men of GOW are ugly and heroically-proportioned. While a male figure can get away with that, a female can not. You will want this armor to fit your sister well so that it can not only be bad-ass, but also fit her in a more "graceful" way. -As shown in the screenshots.
With method 2, the duct tape mannequin is cheap and easy. Get a few rolls of duct tape and some form of stuffing and you're good to go. (Google "duct tape mannequin").
With a perfect torso of your sister you can size-up and adjust pieces so that they fit and flow correctly. Use sheets of paper for quick shape mock-ups, then when that works, trace it onto either poster board or card board. You'll use lots of scotch tape for this. Once you have all the armor made in card board or poster board. Take it apart, and use the pieces as templates for the foam pieces.
I'm using method 2 on my armor now, and I really like it. It takes the "chance" away from building some one else's pep armor which was created from video game characters that have extremely long legs, short torsos, broad shoulders and barrel chests. The amount of re-fitting it would take to fit a real male, let alone a female is not worth it. Best to create new pep files.
-just my 2cents
Good luck!