Hi there and welcome
Do you have access to the wifi router at the sisters house?
ok, the only chance would be to gain access to the router (you can reset it but depending on the make and model of it he may see this) then router the DNS entries to a service such as open DNS
Then you can see which sites he is visiting.
You cannot see this online anywhere unless you have something like this in place.
Whats the make and model of the router?
If logging has been enabled then yes, IF he has not set the password.
If he has then no as you need to reset the router and that would wipe all the data from it.
no, only current sites, you would not be able to get past information if it was not recorded.
you cannot as it was never recorded unless you have physical access to his computer, then you can check the history but you indicated you do not correct?
yes, thats why I indicated its not possible to get historical information, only current information once you alter the wifi router.
depends on the make and model of the router.
they can be changed if the person knows what they are doing.
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