TWO SISTERS SING FOR THEIR FATHER: At the Nashville Center, Mattie and Dani — the two daughters of Alan Jackson — performed a duet of “Remember When” to express their gratitude to the man whose voice shaped so many memories. The room felt quieter than usual — not because it lacked sound, but because everyone seemed to be listening more closely. Mattie and Dani Jackson stepped onto the stage without fanfare. Soft lights. No rush. Just two daughters standing together, holding a song that had alrea