It is explicitly stated in her Before Crisis and Crisis Core profiles on the respective official Japanese websites, as well as her in-game description in Before Crisis and in the profile written in Crisis Core: Final Fantasy VII Ultimania, that Cissnei grew up in an orphanage ran by Shinra, that she was recognized for her potential and was hand-picked to undergo rigorous education and training so that she can become a Turks member in the future. These profiles and descriptions also explicitly state that Cissnei is the youngest person ever to join the Turks, further emphasising her talents and the potential that Shinra saw in her when she was at the orphanage.
Her page in the Ultimania even goes on to mention that Cissnei is a girl bearing a complex upbringing and that she spent her childhood in solitude due to her potential (to be a future Turks member) being discovered by Shinra, and the company, in turn, provided her with specialized education and training for the gifted in order for her to devote her entire life to the Turks. The idea of “anyone other than myself is my enemy” was drilled into her head as part of the education she received from Shinra.6
Throughout the game, Cissnei does reveal bits and pieces of her past to Zack. During the mission in Junon, Zack found Cissnei on the upper level as she finished battling a group of Genesis Copies and proceeded to praise her, suggesting that she'll rise through the ranks in no time and that he would throw a celebration party for her when she gets promoted. Cissnei responded that the thought of anyone ever celebrating anything she did has never occurred to her before this conversation. She also told Zack that she was raised to believe that every person she meets is an enemy initially.
Zack also made a comment to Cissnei that she is about the youngest Turks member that he has met and asked her what she did before she joined the Turks. Cissnei told Zack simply that she received all kinds of training and then she joined the Turks.
Near the end of the game when Cissnei found Zack in his hometown, Gongaga, Zack asked Cissnei about her parents, to which she revealed to him that she was raised inside Shinra.