wait til you have your own place. seriously. good luck.
First make sure that this is what you really want and remember once it is done there is no going back. Then if you are close to your younger brother then ask him to sit down with your parents and yourself tell him that sometimes life is not as it seems and that sometimes both men and women are not happy with the sex that they were born with and it is very hard to live with it so for you it is a chance for you to be very very very happy by going to have a sex change and live life as a woman. Also tell him that this does not change the way you will feel about him everything will be the same as it was before you had the sex change only from now you will be calling you sister and you will still be friends with him and treat him like you used to. I hope that you are happy with your decision and that you never regret it. Good luck to you.
Tell him you were trying to massage your nuts on a belt sander and things went horribly awry. Since he's only six he'll understand that from now on you'll have to wear dresses for one and secondly he'll never try any sort of risky mas-tur-batory technique when he hits puberty. You'll insure he lives a happy life with his nuts intact.