Sisters

Sisters (シスター, Shisutaa), is a title given to female humans who are preparing to become Mamas. They are caretakers for orphans in the Premium Farms like Grace Field House and Glory Bell. Sisters are previous "orphans" who have surpassed an insurmountable level of expertise in the Litany Exams and have spent a lot of time of training. They need to keep up their hard work or they may also get killed. "Orphans" are given the choice to become sisters if they lived to 12 years of age at their farm.

The other option is to be killed and have your body shipped off as human meat for demons. When a girl is picked to be a mom/sister, they are sent off to train and study about what they have to do and know to keep a plant thriving. They are also given tips on how to take care of children, clean, sew, etc. We know this based on the various stereotypical "motherly" activities shown in both Krone and Isabella's training montage.

The mothers and sisters do not have to cook because food is sent to them from outside sources. Like mamas, they have chips in their chest that do not allow them to leave their farm. If they leave farm property, they are immediately killed using the chips implanted on their heart. This creates a seemingly hopeless atmosphere and mood. It is a reoccurring theme throughout the series. This also sets up the lure.

They are supervised by a Grandma that chooses the Top scorers to become Mamas.

Appearances
Sisters can be seen with dresses almost identical to dresses of the English Victorian-era maid style. This is depicted as a long black base dress that goes beyond the ankles, a pure white apron that covers the front of the base dress, a pure white belt that wraps around the waste, and a pure white caller that tucks in under the turtle neck of the base dress. Some dresses also include a series of buttons along the neck-line. These dresses are exact replicas of what Mamas are shown wearing. The dresses are depicted to look like maids possibly to represent the sisters and mamas endless service to the Grace-Field and Glory Bell factory. While training, the sisters are shown to be wearing the same dark grey, long sleeved, uniform shirt. With identical in color, long, and loose sweat pants. They receive the official uniform when sent off to a house to raise, serve, or simply to help out.

Trivia
Their Victorian-era maid dresses worn in both the show in manga by the sisters represent the everlasting service they must do in order to run from what seems like their own fate. They are supposed to feel like servants, doing whatever it takes to meet ends due. Or in their case, doing whatever it takes to avoid death. This creates all the young girls put in this situation to feel powerless and brain dead. As if their only purpose is to help the very farm that's enslaving them.