The therapeutic response is clear. Get the family secret exposed and this can change family dynamics, quite profoundly at times.
The young adult and independence
If she believes this (with or without his encouragement) she finds herself in the same relationship dynamics as the teenager who is not economically separate from the parents. The marriage will then suffer.
The interwoven structure of the life script
Any system is only as strong as it weakest part. In therapy one looks for the weakest part of the five areas and focusses on that.