Date: __/__/____ (date)
Subject: Happy Raksha Bandhan
My dear brother,
I hope you and aunt are doing good. I am also good here.
On this auspicious occasion of Raksha Bandhan, I write this letter to wish you the same. I would like to inform you that I am grateful to God that I have got a brother like you who is caring, and loving. You are my best friend and much more than a brother. We will meet soon.
Take good care of yourself and your family.
Your loving sister,