I'd like to throw some cold water on this idea. First of all, your grandmother's behavior can well have influenced your mother, and she influenced you. So I don't see how anyone can say it is all genetic.

I think the "biopsychosocial" model is most likely correct: there are influences from all of these: one's biological (hereditary) makeup, plus psychological influences from one's family experiences as a child, plus the effects of society -- those cannot be insignificant. People are naturally variable in their vulnerability to outside influences, so whether one becomes "mentally ill" is not due only to one factor.