How serious is your sister about manga? Does she practice alot?
If she just likes sketching in black and white, a new sketchbook is always nice along with some pens. I recomend Sakura Microns pens for this.
If she is really into full colored illustration. I recomend some Prismacolor markers, with a tablet of "Marker Paper" to use them on. Why marker paper you ask? -The markers will last a lot longer and it allows for some blending of the colors. "Layout Marker" paper is also acceptable.
A any decent art store should have these supplies. But if there isn't a good art store near you,
http://www.dickblick.com is where I buy supplies. They have large selections and cheaper prices.
EDIT: You know, Watercolor Pencils would also be an excellent choice for manga. They aren't real expensive either.