Answer:
Dearest [Friend's name],
I hope this letter finds you well. I am writing to you today with some exciting news. My sister's marriage is finally happening! The wedding is scheduled to take place on [date], at [time] at the [place name].
I know you've always been close to my sister, so I would be honored if you would attend the wedding. I know you have your exams coming up, but I was hoping you could come a few days early so we could spend some time together before the wedding.
The wedding will be a traditional Indian ceremony, so please dress accordingly. I know you're busy, but I would really love you to be here. It would mean a lot to me and to my sister.
Please let me know as soon as possible if you can make it. I'm looking forward to hearing from you.
With love,
[Your name]