This would be a good issue for your sister to seek treatment for, especially since it seems to be greatly impairing her social interactions and ability to be gainfully employed. You could suggest to her that she see a psychiatrist
for initial evaluation. The psychiatrist would be able to help determine if the compulsive lying is an isolated problem or if it is also accompanied by other psychiatric symptoms, such as a mood disorder. Based on what the psychiatrist finds, they will be able to make recommendations for treatment.
Isolated compulsive lying typically doesn't respond very well to medications. However, psychotherapy, or counseling designed to help the individual gain insight into the lying behavior, can be quite helpful. Her psychiatrist could point her in the direction of a good therapist
to get this sort of treatment started.
If there are other associated psychiatric symptoms, her psychiatrist will also be able to help her decide whether any medication therapy would be indicated or helpful.
Start by encouraging your sister to make an appointment with a medical professional. This will be her first step towards learning to deal with this problem.