The Sisters of Plenitude were an order of Catkind nuns, devoted to helping the sick (though they were ready to defend their lives with violence). They worshipped the goddess Santori but not much else is known about their faith.
One group of the Sisters (led by Matron Casp) had a hospital on New Earth, outside of New New York in the year 5,000,000,023. This hospital was created before the humans got there and was created to deal with all their diseases. They had cures for nearly every known ailment and were able to heal their patients with remarkable efficiency. Many of the treatments shouldn't have been discovered for at least a thousand years, according to the Tenth Doctor.
Unfortunately, these amazing skills were gained by using thousands and thousands of grown human clones. They were infected with every known disease and killed if they started to show signs of thought. This was due to the fact that other, more humane methods were too slow to deal with all the diseases. These disease carriers were released by Lady Cassandra and eventually cured by the Tenth Doctor. The Sisters were later arrested by the NNYPD for their actions.
One of the Sisters, Novice Hame reformed and became the carer of The Face of Boe (whom she care for in the hospital) until his death thirty years after the events of New Earth and is assumed to have a large role in the governance of New New York and/or New Earth.