okay I should have made it clearer in the walkthrough
The sister's room will be locked until you finish her first quest (returning her phone)
The parent's room will have the following chances everyday:
- Be locked (You can't do anything about it at that time, so just continue on your day).
- Mom might be in the room, so you can't steal her panties.
- The room will be unlocked and mom isn't there, Now you can obtain her panties.
So the only source of panties in the start is from the Parent's room.
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I'll update the walkthrough without relying on the panties next time. Might add one more day to the whole deal, but alas will cause less confusion to the player following the guide I guess.