Sisters

Sisters (シスター, Shisutaa), is a title given to female humans who are aspiring to become Mamas. They are caretakers for orphans in the Premium Farms like Grace Field House. Sisters are former "orphans" who have proven to be capable of having children and have surpassed an insurmountable level of expertise in the Litany Exams as well as spent a lot of time of training. They need to keep up their hard work or they may also get killed. Girls born at the Premium Farms are given the choice to either die or train to become sisters if they lived to 12 years of age at their farm.

When a girl is picked to train to be a mom/sister at Grace Field, they are sent off to train at Grace Field Headquarters and study 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. As seen in the bonus chapter Seeking the Sky of Freedom, most sister trainees are "rejected" (killed) during their training. Very few girls training to become Sisters actually get the position, and even fewer get to become Mamas. The Sisters are supervised by a Grandma that chooses the Top scorers to become Mamas. Most Sisters reside at Grace Field Headquarters for the rest of their lives. Their job includes preparing food and supplies that will be sent to the "orphans", caring for the 0-1 year olds, and giving birth to more children to fill the "orphanages". They are never allowed to meet their children as they are taken away shortly after birth. Sometimes, when a Mama requests extra help, a Sister will be sent to live at one of the "orphanages" to assist the Mama in caring for the children. Like mamas, they have chips in their chest that do not allow them to leave the farm. If they leave farm property, they are immediately killed using the chips implanted on their heart.

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 waist, and a pure white collar 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 to become Sisters, girls are shown to be wearing a 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.