The correct etiquette for sympathy thank yous (for flowers or just cards of sympathy) is to send cards back to the people who gave you either flowers or a card and add a little note at the bottom of...
THIS IS AN EXAMPLE LETTER
Dear Aunt Honey,
Thank you very much for the necklace and matching earrings. I wear them all the time and really appricate you giving them to me.
Thank you,
The Seer...
Usually just a simple thank you. They understand your pain and are showing as much sympathy as they possibly can. Just a thank you can be a good expression of thanks.