Petunia,
I don't think that you will ever have to read this but I'm writing it just in case. If you are reading this it means that the worst has happened, and for that I am sorry. I'm also sorry for all the things that I have left unsaid, and for all the broken pieces of us that I will never be able to piece back together, but there are some things that I want you to know.
The first is that I love you. I know that we haven't been close in years, and likely we never will be but I need you to know that I never stopped loving you. You were there for me and protected me when we were small. I know we were forced to grow up too fast and I know that you tried to shield me from that. You shouldn't have had to do that.
I know we fought but when I think of you I remember the happy days of our childhood. The summer days were spent at the neighborhood pool, or riding our bikes by the creek. I miss those days and everything that came with them. I miss when we loved each other out in the open, when you were the other half of me, completing me in a way only a sister could.
I know I didn't do enough, and I know that I left you in that house. I'm sorry. I don't regret doing it because it led me to who I am. It led me to James and brought me Harry but I am sorry that there wasn't a way that I could find myself and not lose you.
When we were young I thought you were the most beautiful person in the world and I hope that one day you will be able to look back at our childhood and see yourself the way that I saw you. Beautiful and brave.
This war that I got myself caught up in is terrifying, and I can't let you get caught in the crossfire, but Pet I'm scared. I'm scared that I'll die and my life won't mean anything. I'm scared that I lost you for nothing. I'm scared that every terrible thing I thought about me, and you, and us is true.
I know that you pushed me away because you were scared. Scared of us being different, and scared of losing me. I know because I pushed you away for the same reasons. I forgive you for what you've said and what you've done and I hope one day you will be able to forgive me.
If you get this letter it means that I'm dead which means that James and Harry are too. I want you to know that if I'm gone and they are too you are the last piece of me. I think that you'd be sad if that happened because I would be sad if I lost you again, but I don't want you to be sad forever. I mean to be sad for a little bit, you know I always loved the attention, but don't be forever.
Go and live and be happy. Find what it is in life that makes your heart sing and do that, and when he is old enough I hope that you will tell Dudley about me. Tell him about your magical sister who you sometimes hated but who always loved you. Tell him that I loved him even though I never met him, and tell him how good you were back then. And then tell yourself that even though everything is broken you were always enough for me.
All of my love,
Lily Potter