Your following the walkthrough that comes with the game? If so ignore it and instead follow the
Main Quests Walkthrough that's on the OP. The walkthrough that comes with the game has incorrect/outdated parts.
What you're supposed to do in Maria's Quest after going to school is go to her room to get the keys to your room, is at the object that looks like a printer, and check the laptop. You'll need a password so wait until around 19h and check the living room to find that the laptop is now there. Examine it to know the password (you don't need to write it down since your character will memorize it). The next day, after waking up, re-examine the laptop in Maria's room then try to leave the house and she'll order you to do chores for her. Do the chore that she asked after school (it's random). After doing them for the next 4 days she'll take you to her sister.
The answer is in the following
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