Dr. Sosa (Nicola Bartlett) was the tertiary villainess of the 2018 film Dark Sister. She was introduced as the therapist of artist Cassie, helping her as she finds herself tormented by dreams depicting murders.
In actuality, however, Dr. Sosa was the mother of both Cassie and her long-lost twin sister Diana (the film's main villainess). As revealed by Cassie's "father" Hector, Dr. Sosa had been researching human emotion and the possibility that one sentiment being having the power to control it. As part of her research, Dr. Sosa performed experiments on her daughters, among other patients; including Hector (who was not actually the father of her children) and his twin brother, Apollo.
As an infant, Cassie was taken from her mother's stranglehold by Hector fleeing with as a fire broke out in her home, leading Dr. Sosa to concentrate her experiments on Diana. The film's events have Diana having escaped her mother's clutches, turning heel by using her powers to become a serial killer. As Cassie began to realize her dreams were connecting her to the serial killer, Dr. Sosa began to reveal her true colors during a meeting with Cassie by trying to give her an injection, possibly intending to recapture Cassie for more experiments. After Hector and Cassie's attempt to use an emotion enhancement drug to defeat Diana left her bloodied and weak, Dr. Sosa abducted her daughter and brought her back to her lab, restraining her to a table.
The evil Dr. Sosa briefly torturing her daughter with electric shocks in punishment for her escape, while also threatening to continue using the torture if Diana didn't "behave". Dr. Sosa later told Diana that she intended to impregnate her with a child that would "change the world"; likely intending to use said child in more sinister experiments. Dr. Sosa then had Apollo have sex with the captive Diana, smoking a cigarette while coldly watching her daughter's rape and shocking her again after seeing her whisper to Apollo. But shortly afterwards, Hector and Cassie took over their respective twins' body, with Diana attacking her mother with a knife and tying her to her chair. After accusing Cassie of (in her mind) being a bad influence on Diana, Diana killed Apollo with a stab to the neck. Dr. Sosa then attempted to manipulate her daughters against each other, claiming Cassie was jealous of Diana and wouldn't give her the answers she wanted, while sarcastically claiming Diana was not at fault for her killing spree. Unconvinced, Diana went about killing her demented mother by strangling her to death with a wire.