Now I Found You by Mila Oliver

She thought her little sister was dead. But she's about to find out that no secret stays hidden forever... A captivating and suspenseful domestic thriller about betrayal, love, guilt, and the twisted secrets families keep. Seven years after her little sister drowned in a lake, Kate is finally beginning to heal from Emily's tragic death. Until one night, while back in her hometown of the Catskill Mountains, she sees a familiar face. It's Emily. The girl is all grown up now, but Kate knows her sister's face better than anyone. The chilling sighting reignites the doubts Kate has always had about Emily's drowning, and forces her to revisit all the mysterious circumstances that surrounded that day. With the help of her hometown ex-boyfriend, Kate desperately tries to track down the girl she saw. She discovers shocking secrets from the past, confronts her own guilt from that day, and faces buried demons from her childhood. The more she digs, the more she becomes obsessed with uncovering the answer to one question: What really happened to Emily?