Abby Chadway is a villainess from "Deadly Mirage," episode 8.20 of Diagnosis Murder.
She was played by Challen Cates, who later appeared as the evil Fay Gussman on NCIS.
Abby Chadway is a seductive con artist who posed as the twin sister of murder victim Lily Wilson, Steve Sloan's former girlfriend. She first encountered Steve Sloan in the beginning of Part 1 of the episode, and her resemblance to Lily bewildered Steve. His attempts to talk to her were always thwarted, but when they finally chatted, Abby stated that she came from Boston. It was revealed by Mark Sloan that Abby spent her time in Boston committing insurance fraud; posing as the sister of an accident victim. She managed to cash in handsomely, and among the pieces of jewelry involved in the estate was ankle bracelet that was worn by Abby.
After pulling that scam, Abby had seen Lily on a dating show, which is how Lily and Steve met. She targeted Steve and convinced him to look into the possibility that she and Lily could be related. As planned, the documentation stated that she was Lily's sister as well as the sister of Frederick Wilson, a phony doctor who murdered Lily in order to gain control of the estate. Her plan included getting Frederick finally convicted and possibly executed so she could take both Lily's share as well as Frederick's. She phoned Steve regarding Frederick attempting to leave town on a plane, and then attempted to manipulate her way into Frederick's good graces.
Shortly after Frederick's arrest, Steve confronted Abby, who confessed to her scheme. The scheming villainess attempted to coerce Steve that she had changed her ways, but to no avail; Steve ended up arresting Abby for insurance fraud.