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. Shortly after Deanna's birth in 2336, 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. She even destroyed everything that was a reminder of Kestra, but Mr. Homn secreted away a picture of her in case Lwaxana wanted to remember her in the future.
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")