Wireless Connection Issue

[HalcyonSpirit]

My sister uses a Linksys wireless adapter (g-standard) to connect to the router at home. Most of the time, it works alright. However, when I come home for break (like right now) and connect my laptop to the router (regardless of it being by wireless or Ethernet), my sister's computer can no longer connect to the router reliably. It'll usually connect for a few minutes before losing it again for quite a while. When I leave, things go back to normal.

I don't have any connection issues myself, the family computer doesn't have any problems either (it is connected via Ethernet cable), and my other sister's laptop does not have significant problems connecting to the router wirelessly (it only sees "Limited or No Connectivity" for a short time after initially connecting regardless of whether she is upstairs or downstairs; the router is downstairs). It shouldn't be the router itself, since this has been happening with both the current router and the router we had previously. I can't see why this would be a problem with the computer either. In fact, I can't see why this is happening at all. Any ideas would be helpful.