Sylvia Sinclair and Selina Kyle were childhood friends. They also worked the streets together as prostitutes. Sylvia was there for Selina's "first time" and even took her place so Selina wouldn't have to go through with it.
Catwoman (Vol. 3)
In Catwoman: Final Report, they go back to the past, discussing who helped her during her prostitute time. Selina didn't know how to handle Sylvia after the incident which lead to Sylvia's hatred of Selina because she felt Selina abandoned her. Years after Selina became Catwoman, Sylvia still never forgave Selina.
In Catwoman: Relentess: Part 1, Catwoman traces a group of small kids who are petty thieves. Once they run back to a church, Catwoman meets Maggie Kyle, her sister, whom she hadn't seen since Stan's death. She talked about how she eventually left to find a new life and explore the world. Moving to the West Coast, she met her future husband, Simon Burton. Years later, Simon's work transferred them to Gotham City.
In Catwoman: Relentess: Part 2, Selina invites her sister, Maggie Kyle Burton, to a dedication ceremony. Bruce Wayne dedicates the opening of the East End Community Clinic which was funded by the donations of an anonymous donor, which was Selina. That night, the building is blown up, apparently by her old friend, Sylvia Sinclair. Yet, it ends with Simon Burton, Maggie's new husband, being kidnapped.
In Catwoman: Relentess: Part 3, the reunion was short lived, however, when Maggie was kidnapped by Black Mask, and brought together with her husband, all in an effort to hurt Catwoman. Trying to help Black Mask, Sylvia Sinclair comes in and tells him how to get at Catwoman.
In Catwoman: Relentess: Part 4, although Maggie was rescued by Selina, it was too late. She had been tortured and forced to eat her husbands eyes. Simon died at the hands of Black Mask. Sylvia Sinclair showed her face before their escape, since she wanted to get back at Catwoman for betraying her.
In Catwoman: Relentess: Part 5, after this failure, he turned his attention once again to Catwoman. Threatening her friends and family, he coaxed her out for a confrontation. He was under the impression that she would enact her 'no kill code', however she did not, as she shot him in the head and blew off his jaw, killing him. Sylvia Sinclair was killed as well. Catwoman took these lives for her sister and her husband. However, it is shown that Roman Sionis survived the attack.