Have a look at the books in the living room to get somethng that can work like a cleaner.
If you mean the door on the left next to the stairs ... use a hook that you find somewhere to unlock it.
I hope it helps. For the first half there is a walkthrough on YouTube further down in the comments.
Part two is here:
I must admit I missed the hook but the book one is misleading.
Also, that playthrough is only like 1 step ahead of me. Your sister showing up should be flagged somehow, otherwise there's no reason to go in that room...
I found a walkthrough via Gamejolt though. Thanks.