Twins - Or Not?

Riddle: There were two girls who were born on the same day, the same month, the same year and the same mother but they were not twins. How is this possible?

Answer: The girls were two from triplets. Source: https://www.riddles.com/9038

There were two girls who were born on the same day, the same month, the same year and the same mother but they were not twins. How is this possible? — Twins - or not? by Jency Lim Riddle