I wonder if these genetic studies of ME/CFS running in families take into account and control for the possibility that, as well as sharing genes, family members living together for many years may also share viruses. If a chronic viral infection is only slightly contagious, it may not very easily spread to individuals from outside the family home, but over years of household contact, such a virus may spread to many family members in the home.
The late Dr John Richardson, a GP from the Newcastle area in England, spent 40 years taking comprehensive notes on his observations of enteroviruses spreading among family members that attended his practice, the virus giving one family member perhaps ME/CFS, and another member heart problems, and another perhaps some autoimmune condition, and so forth.