To answer your question: there may be a genetic factor, but in my literature review, the research on genetic factors was conducted primarily from parents to children; I did not see anywhere that it "skipped a generation" (I will keep looking into this question for you).
According to MedlinePlus, "the cause of antisocial personality disorder (also known as "sociopathic personality disorder") is unknown. Genetic factors and child abuse are believed to contribute to the development of this condition. People with an antisocial or alcoholic parent are at increased risk" for developing this disorder.
According to the Mayo Clinic, "Some research suggests that you may have a genetic vulnerability to developing antisocial personality disorder and that your life situation may trigger its actual development."
Do you know any details about your fiance's mom's disorder...when, how, why it began? Would your future children have any contact with her? Do you or your fiance have any influence as to her mother's current situation (is she under the care of someone?)