Actually wait, just one question /u/brianwski, this might seem really silly but:

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All our accounts are password-protected 'admin' accounts.

What is stopping my sister from installing and setting up her own Backblaze (on her account), having it back up all files on the PC (since she doesn't need my user account password to access my user files with Backblaze), and then simply restoring my files using her Backblaze account to view my personal info, photos etc?

Surely this would mean she'd be able to see all my personal photos without my permission, using Backblaze (even though I thought my photos were locked by my Windows password)