Sam and his stepmother Elaine, Kevin's much younger second wife, have something in common: both of them have lived in a state of ignorance about the existence of the necromancer world. The only difference is that Elaine is still living there, and likely will be for some time.
"Elaine was normal," Sam describes. "Plain ol' vanilla human" (16.33). It only makes sense that Kevin, ashamed of the family necromancy gene, would go for the most normal, average person he could possibly find as a way of starting over. And obviously, Kevin has told her nothing about his past. Ignorance may be bliss sometimes, but it's hard not to feel at least a little badly for this lady who's raising two necromancers and doesn't even know it. Fingers crossed they're more like Sam than Douglas.