Start the game.
Sit in the dining room.
Don't ask mom to let Sandra stay. Sandra leaves. Talk to mom.
Next day, talk to Lucy at the pool.
Tell her what was said.
She sends me in to talk to Sandra.
Tell Sandra I told Lucy "what mom told us yesterday."
She asks how Lucy took it, then goes to make food.
Problem? Sandra was not there for the conversation, and I never talked to her. She should have no clue what was revealed.
This is a massive plot hole, right at the very beginning. If you write a game where a choice alters a scene, you need to take that into account while writing subsequent scenes. Sandra should know nothing.
Speaking of choices... when Mom told you her reasons why?
You should have the option to bluntly tell her that you don't believe her. I'm not a fan of the idea that you just buy her reasoning with no push back. I'm sure the creator's canon for the history before the game starts will say that the mother's depictions of events is the truth, but it would be nice to have the option to at least doubt her, to her face, instead of the choice of "believe" or "do nothing."
Also, bizarre clothing change on mom if you choose to do nothing.