Heading: Introduction
Begin the letter by greeting your friend and introducing the purpose of the letter. You can start by mentioning how long it has been since you last spoke.

Heading: Details of the Marriage
Next, include the details of the marriage. Mention the date, time, and venue of the ceremony. You should also give your friend an idea of what to expect on the day, such as the dress code or any special customs that will be observed.

Heading: Invitation
Make the invitation to your friend by requesting that they attend the wedding. You could say something like "We would be honored if you could attend our sister's wedding as our guest." Mention that you would love to have them there to celebrate the special day with you and your family.

Heading: Accommodation and Travel Arrangements
If your friend lives far away from the wedding venue, you may want to offer them some assistance with travel arrangements or accommodation. You could mention any nearby hotels or suggest that they stay with you or another friend or family member.

Heading: RSVP Request
Don't forget to ask your friend to RSVP, so that you can make the necessary arrangements for their attendance. You could include your contact details, so that they can get in touch with you if they have any questions or concerns.

Heading: Conclusion
Finish the letter by thanking your friend for their time and expressing your excitement about the upcoming wedding. Wish them well and let them know that you hope to hear from them soon.

Remember to proofread your letter before sending it, to ensure that it is error-free and easy to read.