My Dear Miss Evans,
What a delightful surprise it was to receive your letter. It's rare that a sibling of one of our future students takes such an interest in Hogwarts; I'm certain that your sister will appreciate your concern. I am, however, disappointed to have to tell you that, as a Muggle, you would not find Lily's school of particular use to you, despite the fact that, as you so kindly put it, your sister "could stand looking after."
I assure you, my dear, that Lily won't be lonely for long, if at all; at Hogwarts, students are Sorted into the houses of Gryffindor, Ravenclaw, Hufflepuff, and Slytherin. Once Sorted, your sister will have the benefit of the company of her house mates, who will serve as a substitute family for her. I'm confident that she will make friends easily, so you need not worry for her on that score.
As far as your desire to learn the arts of witchcraft and wizardy is concerned, I suggest that, as only the magically inclined may do this for practical reasons, you apply to Lily to learn more about the history and traditions of our world. That said, and because I appreciate and understand your interest in knowing more about Lily's future life, I've enclosed a copy of Hogwarts: A History with my reply. I trust that you will find it fascinating. More than that, I hope it engenders within you a love of history in general. How intriguing you will find it, I believe, as you read the book, to discover the interconnectedness of the magical and Muggle worlds. Why, you may even, by virtue of diligent study, come to know more than your sister of the magical world. Perhaps you'll wish to help Lily study over the holidays. That would make you a very good sister, indeed.
Now then, rest assured that we will take great care of Lily, and know that I, personally, have nothing but respect for your kind wishes on her behalf and your well meant offer to accompany her to Hogwarts. Best of luck to you as you engage in your own studies. I imagine that you're in possession of many fine and special gifts, yourself, and that you'll go far in life, given your inquisitive nature and strength of spirit.
Hogwarts School of Witchcraft and Wizardry
P.S. I knit, myself, and certainly find the pastime a magical one, as well.