When addressing a sympathy card, the writer can direct it to the closest surviving relative of the deceased person. Typically this is likely to be the widow or widower or the deceased, or the eldest of the deceased's children. It is always appropriate to send a sympathy card to any other family member of the deceased.
The envelope of a sympathy card should always be addressed formally, using the titles of the deceased's family rather than just first names. If the card is being sent to the deceased's entire family, it is appropriate to list the widow or widower formally on the first address line, with the children's first names listed on the next line.
In writing a sympathy card to a family member or close friend of someone who has recently died, it is important to make sure the person receiving the card knows who is sending it. In the confusion and sometimes chaos after a death, even a widow who knew all her husband's friends may not be able to make an easy connection to understand who is sending which card. For this reason, it is crucial to sign a sympathy card with both first and last name, or with the names of all the family members sending the card. It is appropriate and helpful to write a sentence or two reminding surviving relative how the deceased knew the person sending the card."