1) Gets scapegoated and blamed for a family's problems
2) Has emotional problems that are not a mental illness, but a normal response to the stress of dealing with an unhealthy family in denial
3) Blows the whistle on a dysfunctional family's problems
Phrase originated because family therapists recognized that the child "identified" as the patient is not necessarily the one who is sick.
2) Becky is extremely depressed and fearful. She accuses her father, correctly, of molestation, but the parents deny it and accuse Becky of being sick for reasons that have nothing to do with them. Becky is the identified patient.