Dear (state name),
I hope you are doing well and adjusting well in your new hostel. I wish to announce that your sister (mention name) has secured (mention rank) position in the (mention name/standard) examination 2010. (The first para must directly and in the very beginning state the announcement which is the objective of writing the letter. If it is a personal announcement, one can begin with salutation and asking about the recipient’s health and well-being).
These results will help her secure a seat in the top engineering colleges of the country. (The second para may elaborate on the announcement and its affects).
Your sister’s result should serve as an inspiration to you and I hope you study well and get good marks in your exams too. Do take care of yourself. (The letter may be concluded with a message of understanding and optimism to the announcement).