This family condolence letter can be given to someone whose in-law has died.
I heard the terrible news that {in-law's name} died. I am so sorry for this trying time for you and your {wife/husband}'s family. I remember when you and {spouse's name} got married, and {endearing memory or anecdote about the in-law}.
I know that {he/she} was a loving {father/mother} and a fantastic in-law. You and {spouse's name} are in my thoughts and prayers.
