The Surgeon stories:
The Romanian Sisterhood, like the majority of the Animalian military students, is a largely student gang which consists of 20 teenage Romanian girls aged between 16 and 19 years of age.
They are typically recognized by being draped in leathers like motorcyclists except for younger members who are typically dressed in different clothing except for Defenders of Earth.
The sisterhood is usually led by Bianca Antonescu but after they come under threat from WOTAN in its attempts to take over the world once again, the girls are led by the Surgeon's new companion Amanda Mycroft who manages them surprisingly well.
After the events of Thunderbolt, Antonescu becomes the group's leader again. While the Sisterhood makes no return in any other literature outside of the Surgeon stories, the ages of the girls are also the ages of twelve more British girls who are taken prisoner by the students in The Mammal Wars and are used to increase their numbers.
Unlike in Thunderbolt where the girls were shown as being like a biker gang, the Sisterhood that returns in Defenders of Earth is shown to increase the numbers of the Romanian Vixens because the Sisterhood is part of the Vixen's youth section even though it very rarely recruits children alongside its enemies.
Because of this, the Sisterhood are the only characters from the Surgeon stories to also appear in Defenders of Earth (The students from The Yeti Factor also are brought back but they are used for The Mammal Wars alongside the Women of the Night). Also in Defenders of Earth, Bianca is the Sisterhood's leader and stays in that position without ever being replaced.
Of the girls, the ones depicted in Thunderbolt in the form of Bianca Antonescu, Camelia Avramescu, Cornelia Brasoveanu, Cristina Barbulescu, Emilia Dimitrescu and Nicole Marin are first shown in The Dragon and the Snake as PHANTOM prisoners while the remaining fourteen girls are shown much earlier; about as early as Zhakuh: The Eight Head Monster, in fact. The aspect of the six girls shown in The Dragon and the Snake is also used much earlier in the story Kilarth where PHANTOM hold the Serbian Vixen Olga Petrov prisoner.