As a general rule, it is appropriate to write a friendly letter that lets the sister know about life's happenings, activities and interests while asking about her life. The type of letter a person writes to her sister varies greatly based on the siblings' ages and relationship.
Write a letter of appreciation to a sister to thank her for all that she's done. Hand-written letters are special because she is able to read your words over and over. In order to write a good appreciation letter, be as specific as possible. For example, instead of just thanking for her support, thank her for attending sporting events or helping with homework.