Ms., Mrs., Mr. and "Dear" are acceptable to write before the names of the people close to the deceased.
One line of an apology or non-offensive sentence is enough, but more is acceptable. The details of an illness, how the death of the deceased occurred and to suggest the death is a "blessing in disguise" should be omitted. A positive memory of the deceased is also acceptable.
You can ask for or suggest a way to help the party affected by the loss.