When writing a sympathy thank you card, you can be very brief. An example of a sympathy thank you card message is "Your kindness and sympathy will always be held in grateful remembrance."
"Your kindness and sympathy are appreciated as we mourn" is another example of a sympathy thank you card. A slightly more affectionate and longer sympathy thank you card example is "Never are we more aware of how much friends and family mean than at the time of bereavement. Your kindness and sympathy will always be remembered." One last example is "We deeply appreciate your kind expression of sympathy in our time of great sorrow. Thank you for keeping us in your thoughts and prayers."
When signing the sympathy thank you card, you can sign the card "the family of . . ." instead of writing everyone's names. Additionally, if you are thanking someone for a donation made in the deceased's memory, do no state the donation amount. Instead, thank the person for the generous gift or contribution. You may also want to specify that you appreciate a gift or meal or flowers if someone has contributed something during your time of mourning. You can often buy preprinted sympathy thank you cards as well so that you do not have to do extra work during your bereavement.