I am just guessing, and the corruption is clearly stronger on mom's side. But if you try to imagine this scenario on a real world level, it would be more than strange if Alexis never had seen Davide and never heard or saw anything suspicious going on in the house. This house is not a haven anymore, even if you try you couldn't shield your daughter from getting an idea what's going on. Again, I don't know what saddog really wants to do with his game or how he sets the scenario in detail, but from a neutral point of view Alexis should be well aware what her family has become, bringing her either to the point of full resistance (no signs for this) or curiosity (very likely for a teenage girl).