Why is the surname Nie?

Her surname is Nie, not Ding, because her father's surname is Nie. In Jin Yong's martial arts novel Tian Ba Long Bu, Nie Feng, Nie Fei's father, is a martial arts expert. He once changed his name to "Nie" in order to escape the pursuit of the government. Later, Nie Feng married a woman named Azhu and gave birth to Nie Fei. Therefore, the surname is Nie, not Ding.