Forget theology of the body, are you mad?! That's very, very inappropriate for teenagers, let alone children and adults.
Why not buy her the two books of Modern Saints, their lives and faces, by TAN books? I have them both and are a very, very fun read. She'll appreciate them more than even the Bible or a Breviary as she will find the latter two rather boring at first.
I suggest also buying the Life of Saint Bernadette by the same publisher and the Story of a Soul from sistersofcarmel.com/.
I also suggest buying he a beautiful Rosary and scapular. I really suggest you stay with the poor little thing and give her alot of support. Teach her about purity and modesty that she may grow how to love it. Do not allow her or encourage her to wear indecent clothes. Forget short skirts, jeans, pants, sleeveless shirts or blouses or blouses with straps. Also, do not allow her to wear that horrible thing called the 'bikini' for summer. That is the number one killer device there is.
However, if your fiancee's cousin insists on going swimming, i suggest searching on something called the burkini(or burqini). It's a modest swimsuit which seems perfect for preserving modesty.
Another thing... get her to love the Blessed Virgin ALOT. I have a big love for Maria Teresita Quevedo as she was such an ordinary girl yet she had such a devotion towards the Blessed Virgin that I instantly became a devotee of this lovely little girl. And be nice and gentle towards your fiancee's cousin. Do not be harsh or anything. And especially, and this is a mistake which many do, do not speak of holy things or of Mary in a serious, moody and sort of angry tone of voice... neither must you speak in a 'MYSTICAL' sort of way. Don't speak for the sake of speaking, says Saint Francis de Sales; speak so as to drop the honey of devotion and the love of Mary in each other's ears.
And another thing... do NOT let her in that High school musical or that Miley Cyrus disney rubbish. Do not let her read Twilight either. We don't want to endanger her purity!