It's kinda hard to imagine the scenario in real world level. What kind of father and husband would put his family's lives on the line for the sake of his job? And as for the newly arrived only son, his first action would be to get his mother and sisters out of that dump immediately and leave that stupid overly ambitious step-dad to rot and die.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).
Following saddog's scenario about Alexis, she was characterized as an innocent, sweet and obedient girl. She wasn't even curious about sex until her brother exposed her to different porn genres. Being the obedient daughter that she is, it was easy to keep her from the secret happenings in the house. Although it is inevitable that in time Alexis would discover the fucked up workings in the house and she will eventually be corrupted and succumb to it with or without the MC's manipulations.
Btw, I tried a playthrough with Alexis with 0 corruption (So with the other women). It was boring as hell but the outcome was that she sticks to the MC like glue... innocent love and all and she didn't get interested or curious about Davide in their first meeting. So yeah the love route does develop into a story where the MC is in protector mode and somehow keeps the other assholes at bay.