First of all, when you are writing a letter you should write your full address, including your zip code and then the address of your sister. You should also write the date and then you can start with:
Dear sister Maya,
I am writing to you because I know that this is a hard time for you. You are strong and you should be aware of your good manners.
Our parents were always proud of you and they trust you a lot, you should trust them too. I think that you must sit down with yourself and think about this situation.
You were always special and smart, so I am suggesting you act smartly now. I know that you want to act like that but don't. I think that it will be best for all of us.
