Explanation:
If you are writing a letter, you should first include your full address that includes your name, street name, country, zip code, and other.
You must also write the address of your sister and the date of writing.
After that, you can start with something like this:
Dear sister Sandra,
I am writing to you because I know how difficult this situation is for you.
My opinion is the same as the opinion of our parents and I know that in the future you will think like us too. Now you are young and you do not know what to do.
I will give you advice that you should not act like you want because of the situation and you should trust our parents. Do not act like a young person, act like a mature one.
Sending lots of love!
Maria M.