[Your letterhead, if desired; if not, your return address]
[Date of letter-month, day, and year]
[Recipient's first and last names]
[Street or P.O. box address]
[City, State ZIP code]
Dear [recipient's name]:
Where do I begin in attempting to express my deep appreciation for the caring you have shown to me as a sister? Over the years, we have grown by being close sisters, and now best friends. I appreciate so much about you and our relationship-your discerning advice, your encouragement, your humor, and so much more. I hope I am to you just as much of a support as you are to me.
Thank you again, sis. Words are not adequate to express my gratitude, but I hope you can get at least some sense of my profound appreciation.
[Sender's first and last names]