Kestra Troi was a half-Betazoid, half-Human girl, born in 2330 to Lwaxana and Ian Andrew Troi. Kestra was the older sibling of Deanna Troi. Kestra was picnicking at the water's edge with her family and the family dog. When Lwaxana looked away for a second, the dog and Kestra ran off. Kestra fell in the water and drowned.
The memory of Kestra's death was so traumatic to Lwaxana that she buried the memory deep inside instead of dealing with it. In dream sequences Lwaxana was forgiven by Kestra, and in turn herself. Only then did Lwaxana find peace from the tragedy. (TNG: "Dark Page")
When Lwaxana became pregnant in 2372, the thought of losing her child to the Tavnian culture made her depressed, stirring the pain of Kestra's death. She would later confide to Odo that the losses of her parents, sister and husband, did not compare to the loss of Kestra. (DS9: "The Muse")