Date: __/__/____ (date)
Subject: Happy Raksha Bandhan
My dear brother, I hope you are doing good. I am also good here. I am grateful that I have got a brother like you who is caring. You are my best friend and much more than a brother. On this auspicious occasion of Raksha Bandhan, I always pray for your long life.
I wish you Happy Raksha Bandhan and eagerly waiting to meet you. Take good care of yourself.
Your loving sister,