My sister is 14, and she's a semi-professional, beyond-her-years writer. She has written 70,000 odd words, and is working to study for a BA (hons) within the year. She knows everything about literature, words, grammar. And her novel is hitting on touchy subjects such as rape, romance, sex and family abuse - she does so much research, interviews, and all.
I also think her novel is the standard of many great authors, and although being her sis I am a little biased, loads of other people have agreed (people who aren't being biased).
I also think her novel is the standard of many great authors, and although being her sis I am a little biased, loads of other people have agreed (people who aren't being biased).

But it took a lot of time to find the right resources to work with... Now I am 21 and finally able to record songs :) it does happen.
now, after having done away with the futility of your question...to the content itself.
I think many people are writing SOMETHING, sometime during their puberty/early adulthood.
Now, the question is not even if your family or other, neutral people think it's great...
Of course it speaks for her not to avoid certain subjects, and that she's doing research on the things she writes...but the opinion that matters is that of publishers. And the question in-how-far she can create realistic scenarios or sticks to scenarios that she is familiar with...(say, a work environment may not come across as realistic if she never worked/been in one)
Is there any reason you did not try for a book contract yet and see if she can get it published?
I am not trying to sound nasty here, sorry if that comes across so...but you should go that extra step and see what THEY say-...here it just seems like "Listen all, my sister is so great!" but ultimately nothing we say will help her with a breakthrough.
Support her, as it sounds she's doing a great job, but support her properly and get her published, or at least try to!