What Can I Write For Sympathy Messages?

Just empathise with the person and let them know that you are there for them and that you care..

I want you to know that I am so sorry you are having to deal with such a great loss.   I know you were extremely close to your sister, Emily.  She was always very kind to me.  I, too, shall miss her.  
Sincerely,
Frank Jones

I would like to extend my sympathy to you in the loss of your sister, Emily.  I know you shall miss her terribly.  She was a kind,  thoughtful lady.  I shall never forget her kindness to me when I lost my Father.  She was a brilliant choir director and I am sure that today she is directing the angels in Heaven.
In deepest sympathy,
Frank Jones

You sister, Emily, will be missed by everyone she directed in the Elm Street Methodist Choir.  She was a good Christian and demonstrated it in all of her dealings with us.  I shall miss her and I know you will also.  I would like to extend my sympathy on the loss of such a wonderful person.  My God bless you, give you peace and ease the burden of your loss.
Sincerely,
Frank Jones

I hope one of these can be edited to your satisfaction.