This is after the movie At World's End and will spoil it if you haven't seen it.

Chapter 1: The Letter

Dearest Brother,

I am sorry to say that I must press upon you another problem. My child is in need of care. I am going to die soon, so when you get this I hope you will understand why there is a teenager on their way to live with you. Please raise my child accordingly and if there is a woman that can help you then please let them. I would go to someone else, but with my condition and how late it is I come to you begging. By the time you get this I will be dead. Take care of Sam.

Love,

Your Sister, Sarah

P.S. Jack don't do anything stupid or be a bad influence.