The most common reason my Internet connection gets slow is because someone else on the network is streaming video. Didn't you guys recently get Netflix? I'd recommend checking to see if your parents are streaming movies...lol
I'd also recommend looking up your router's manual online and finding out how to see what devices are connected to it to make sure neighbors haven't been connecting to your wireless.
Sometimes I'm not sure if it's my Internet or my computer that's slowing things down. On occasion I'll got to http://www.speedtest.net/
and do a quick test to see where I'm at. It doesn't fix anything, it just gives me an idea of how fast my connection is.
Finally, and maybe most importantly, turn off all file sharing software and scan your computer for viruses, spyware, and malware. I'd recommend Spybot Search & Destroy and/or AVG Free; there's other software out there, but that's what I personally use. You may want to do this will all other computers on your network, including your parents'.
Sometimes, when everything is said and done, it just boils down to a sucky Internet Service Provider(ISP) that's spreading itself too thin around your neighborhood. I hope that's not the case; you can't do much about that problem (except switch ISP's, but they often setup monopolies in residential areas, like mine
So yeah, to summarize:
1 - Check to see if other computers on your network are sucking bandwidth
2 - Check to see if neighbors are stealing your Wi-Fi Internet
3 - Do an Internet speed test to make sure it's the Internet lagging and not your computer's processor
4 - Turn off file sharing software and scan for viruses