Mei (Seihou) & Mai (めい＆まい) are 6-year-old identical twin sisters who were orphaned at birth. They are known as the "Death Twins" and they have appeared in all of the Seihou Project games. Their age as of Kioh Gyoku is 5, and in Banshiryuu they're a year older.
https://en.touhouwiki.net/wiki/Mei_and_Mai