In 2004. Sister Fangfang 12 only had three birthdays. Sister Fangfang should have been born in 2004, and 12 divided by 3 equals 4, which means Sister Fangfang only has a birthday every four years, which means Sister Fangfang was born on February 29th of a leap year. 2004 divided by 4 equals 50 1, 2004 is a leap year, 1990 and 2006 are normal years. Then Fangfang's sister should have been born in 2004.