Date: __/__/____ (Date)
I hope you are doing well and I want you to know that we all are fine here. I hope your _________ (studies/job/other) are going great and let me know if you need something.
I am writing this letter to wish you a very Happy Raksha Bandhan. You have always acted as a supporting pillar for this family. May God bless you with great health and happiness. I am trying my best to visit during this festival but it is very difficult as I am occupied with _____ (office work/business meetings/other). My blessings are always with you.
Take care of yourself and send my warm greetings to everyone in the family.