|I agree immensely with everything Whisky P has posted above, and can only add the suggestion that, instead of lending/forcing-on your sister specific books, leave them around. Leave them (them could be specific books attending the slant of feminism you'd like to enlighten her on, or, well, random stuff she might like but is unlikely to pick up on her own - the Ken Kesey, the odd Kathy Acker, or children's books, even; things she might well enjoy but could plausibly be too embarrassed to try pulling from a bookstore or library herself) behind when you split, somewhere she's likely to pick them up if she's intrigued by the cover, title, or the blurbs and synopsis on the back. She's less likely to feel she's being guided or educated, which is always uncomfortable, no matter how well-meant, and she'll be selecting the things she wants to dip into on her own, finding her own tastes and tendencies.|
Thinking on my own reading at fourteen, or thereabouts, I'm recollecting the prose of the Brontes, the essays of bell hooks, and the poetry of Ann Sexton. (Whom are all mentioned above, I believe, which is nice.) And Philip Farmer's fictional biographical tales and analyses of Doc Savage and Lord Grandrith. (Which are not mentioned by previous posters.) If you have deep issues against thinking of your sister reading excitedly about pulp heroes past their prime and pushing their immense hard-ons together with the fate of the world hanging in the balance, or rhythmic relaying of the rape aspect of that infamous Sleeping Beauty tumble in the bramble, you might want to pass on some this, though she may not.
I'd be more concerned with when she reads this thread and sees you've noted her as the 'snot-nosed sibling' in need of structure and education. The Valerie Solanas might be less conflagratory.