When you are writing the letter, you must include your full address, date of writing, and address of your sister. After that, you can start.
Dear sister Anna,
Congratulations on your success in the speech competition. I knew that you can make it. I believed in you every second and our parents are very proud of you, and so am I. I cannot describe this feeling of happiness. It is so great to see you happy and succesfull. I think that this is a great step in your future. I love you and I cannot wait to see you again.
