Is cfs genetic?

Poor Understanding. There is a poor understanding of cfs and it's causes. In the future, we may discover a genetic predisposition to people developing cfs. Likely more than one factor involved (i.e. Genetics, infection, environment, etc).
NO. No, it can mimic a family's behavior pattern, but i don't think of any genetic involvement.
Possibly. CFS no longer exists, because it trivialized the condition. This January after recommendation by the US Institute of Medicine, it is now Systemic Exertion Intolerance Disease (SEID), with simplified criteria, one possible cause is mitochondrial disorder, usually genetic with a high degree of variability in a given individual. The key symptom is intolerance of exertion.