Sister Mary was the leader of the Little Sisters and was often referred to as 'Big Sister'. As a vampire she fed off of the blood and pleasure of humans to maintain a youthful human appearance. In actuality she was not much more than a shade. She was also a very strict leader and would often whip those who disobeyed her, or send them to the Thoughtful House.
In The Little Sisters of Eluria, the Little Sisters capture Roland Deschain. Sister Jenna falls in love with Roland and helps him escape. When they reach the outside a confrontation between Roland, Jenna and Sister Mary ensues. Jenna uses her bells on her headdress to call a Cross Dog that tears open Sister Mary's throat and pulls off her head.
It is unknown exactly what type of vampire Sister Mary falls into. However because she seems to be intelligent and has a weakness to religious symbols (ie. the religious pendant that Roland uses for protection), it can be assumed that she is likely a Type One. However, neither she nor her underlings seem to have any vulnerability to sunlight.