if you're really adventurous (and have some spare pc's/NIC's lying around) you can set up a "beefier" router/firewall using something like m0n0wall. In there you can set up QoS which can throttle bandwidth for filesharing/p2p apps to the lowest priority.
That way, when no one else is using anything, your sister can enjoy the full amount of bandwidth she is able to. But when other people are doing more "productive" (and I use that term loosely) things they get higher priority and thus aren't as negatively affected.
But this can be more headache that its worth if you aren't sure what you are doing.