Understandable you feel that way, but I disagree with a full-fledged tablet being the next logical step. If she only draws, getting something like a Spark caters more to line art.
At 15 I guarantee she has a lot of room to learn the fundamentals. Things like perspective, proportions, value, materials, composition, colour coordination...these are all techniques you learn throughout the more maturing years of art creation.
Getting her a tablet now will also introduce some bad habits like relying on undos, using transform tools to fix proportions, and relying on layers to bypass planning out the image.
She's your niece, and of course do what you feel is best, but from an opinion from an artist that has made a career out of this, my advice is to not allow her to dive into the digital realm just yet.
Get her to learn more of the traditional skills...in different mediums. It will strengthen her knowledge and abilities in the long run.
Baring all that, if you still wanna go this route, get her Painter 5 essentials. It's like $50 these days and is fine for painting/drawing.