The prompt says "know you once existed," not "knew you personally." I'm sorry for your family's loss. Even if you can't pass on a firsthand memory of them, you have the power to learn about them and pass on the knowledge that they once existed.

I know that my great-great-great-grandparents August Bergstrom and Anna B. Bergstrom née Johnson once existed. They immigrated to the United States from Sweden in 1881 and settled in Gibson City, Illinois. They had six children, including my great-great-grandfather Claude Otto Bergstrom. I have photos of their grave markers. According to family legend, Anna was to be a mail-order bride but fell in love with one of her traveling companions, August, and married him instead. God willing, this will not be the last generation to know that August and Anna once existed.