Complex sentence: However, the offer was turned down by her parents, reasoning that Krystal was too young and at first only allowed her older sister to join the agency, who debuted as a member of the girl group Girls' Generation.
Split into simpler sentences: However, the offer was turned down by her parents, reasoning that Krystal was too young. Therefore, her parents at first only allowed her older sister to join the agency, who debuted as a member of the girl group Girls' Generation.