The best answer is that lupus is NOT genetic but those with lupus or other autoimmune diseases have a set of genes that predisposes them to some sort of a disruption of their immune response when subjected to some sort of an insult: virus, cancer, bacteria, environmental insult....... In short, lupers do not have a genetic disease which is defined as you have a gene or set of genes and it is 100% certain you will get a given disease. Statistics have shown that AI diseases run in families....member will get AI diseases but not necessarily the same one.
I have polymyositis and there is a myositis disease that is truly genetic called hIBM (hereditary inclusion body myositis) but the other myositis diseases are NOT hereditary.
Hope this helps but nobody really understands how AI diseases work....yet.
Bill