I have ten dollars to my Amazon account, and do not know if I have to take fire for my books or not buy. I? If so, how?

I want to give my original kindle to my sister and buy myself the new edition. But, I also want her to be able to read all the books I have paid for and loaded already on the original kindle. Ideally I would deregister the original kindle before I give it to her so she can re-register to her own amazon account. But then will my old content disappear?