Hello, my sister has a PC which is connected to a modem from our Internet supplier. Recently, I installed a laptop through a router from that same modem. Can she see or is she able to access what I do/see on my laptop from her PC?
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No, unless you grant her access by sharing files/folders, and even then she will not see what you are doing. Just don't do anything illegal because she will probably notice when they drag you away.