My dear little girl, Sister,
I know it's hard to learn you've not been invited to a birthday party. It's not a mark against you. I know it's a hard lesson to learn. I wish I could say that it stops being bothersome the older you get, but I'd be lying. Even as an adult, it hurts to be left out. I think it's human nature.
Please know, my sweet girl, that you are a wonderful young lady. It's a pleasure to be your mother. And, they're missing out on having a fun filled night with you. ;)
Love, Your Mother