This was well written. Some of the use of pronouns got confusing, especially in the third and fourth paragraphs. I think adding to the mystery of Lady Death would be a good idea, as it would make the piece less predictable. Or you could develop her character, making her seem more like a real person, more like one of them, and then making it a surprise that she is actually Lady Death. Also, are the oldest and older brothers the same person? Please clarify. If they are two separate people, give them individual names. Also, you could expand on this, making it a short story or even a book, allowing you to develop the characters more. When the reader becomes more attached to the characters, the presence of death is felt more strongly. Also, it will give you an opportunity to develop characters beyond names, as the number of names and underdeveloped characters gives this piece a little bit of a... lightness,maybe is the word, though a little inappropriate, that an expanded story could improve.I know that you are trying to figure out your own feelings on death in this piece, and my ideas may not fit with what your vision is. And, as this was dedicated to your siblings, this may be much more personal than I know. Please do not take offense at any of my suggestions.
Here are some of my favorite lines:
"In the town of Tirona, near a dry well, a young girl sat hunched over her book as though everything else in the world was meaningless." Great opening line. Maybe you should dwell on that a little more, and also discuss the escape from reality books offer.
I also like the description of Lady Death. You might want to read Jetso's column on description, as I think you could do a lot with it, showing a talent for description already.
Nice job on this piece. Please consider expanding.

A little pridictable, but nicly writtened. I like it.++