The main story is all about the 2 sisters and the daddy hooking up.
If you take them out you are missing over 65% of the story.
If there not family then there is no since about Macy felling so bad about the mothers death.
If there not family then all the dreams are or should be gone or totally re-written.
the main story is not about them "hooking up." It is about how the relationship of the household develops through player choices. Some can choose one path, some can choose another and all can find enjoyment in their preferred choices. Just because that one part is an allegory of the biblical story of Lot and his 2 Daughters doesn'tean the biblical story needs fulfilled.
I disagree and I have played it both as a blood relation household and as a caring foster parent. If a player chooses not to have the MC build intimate relationships with Denise and Macy, sure, they don't see any of the lewd scenes that would be from that intimacy --- but no, players wouldn't be "missing" 65% of the story. Just the lewdity.
Macy still feels bad about how she treated the MC's wife and the events that led up to her death as well as dealing with the trauma after. That doesn't require blood relations. That's called empathy and humanity, particularly about a caregiver's death.
With your contentions, you are dismissing the foster family as a caring, nurturing family. Just because someone doesn't have blood relations doesn't mean they can't be a family.
Also, even with blood relations, a player can choose not to take opportunities with Macy and Denise, and just being a caring loving parent who nurtures their children and allowing them to become they people they can be without having to also having intimate relations with them.
How you choose to play isn't the "right" way to play and the developer has done a great job creating a narrative that offers a variety of play styles based upon player preferences and interests.
Edit: by the way, the nightmare the MC has when his dead wife yells at him for players who choose to have the MC simply ignore the raising of Denise and Macy in any way (meaning not showing them any concern, not even as a foster parent) is one scary scene for those who choose to play the complete asshole.
The developer has done a great job with the narrative to provide a compelling story no matter what style the player chooses - even those who choose no lewdity whatsoever.
