The only genetic concept worth bringing up here is the mutation. Even simple things like the blood types they use as high school examples haven't been fixed since our earliest human ancestors.

One of the standard high school examples of an inherited mutation is hemophilia among Queen Victoria's descendants. Neither she, her husband, nor any of their parents were hemophiliacs, but she got that recessive mutation from one of her parents, and passed it down to some of her descendants.

What I would actually suggest, in addition to telling his wife that his mother and sister are making ridiculous insinuations, would be to get to the root of the matter: "Mom, Sister, why are you trying to destroy my marriage?"