There are two ways for a computer to connect to a wireless network: 1) dynamic (dhcp) - the router assigns a "temporary" IP address to a specific machine that is connecting to the network, or 2) static, where the computer is configured to use a specific address when connecting to the network. This is often the cause of the types of problems you are expreiencing. You need to verify that your sister's computer is using DHCP addressing instead of static addressing. If not, then go to the computer's TCP/IP settings and change it.
rubberman
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