I always thought she had blocked it out of her memory, the fact that she could by lying crossed my mind but the whole bit with the lights and the scene at the end made me think it was a repressed memory.
The Last Boy Scout 5/10 Venom 6/10 The Haunting in Conneticut 7/10 Beyond the Door 4/10
Sometimes when a person goes through something this traumatic they do block out certain, if not all, memories of the event. The lights may have sparked Cynthia to accurately and truthfully remember what happened with Jimmy. Or, she may have just realized, while looking at the lights, that she had to do the right thing and tell the truth about her part in the murders. It's not really clear which one it could have been. Its left to the viewer's interpretation.