Wireless Internet forum: Connection issues after buying new laptop

For Christmas my father bought my sister a cheap laptop from wal-mart since she broke her desktop (lol). Since then we've been having some connection problems, and I'm wondering if it is coincidental and ISP related, or if there is something in her laptop that could be causing all this lag (lol idk anything about this). We don't really get along so I haven't really invaded her PC, but from what I can tell she doesn't download - but I don't even know if she knows what downloading is - but she does stream youtube and stuff. However, I can stream Netflix and youtube and whatever else while she's at school and it is golden, so I don't know if that is the problem.

So anyways, my ping, when it is normal, is 60ms. It rests at that during the day, until about 4:30PM-5PM when she gets home. When my sister uses her laptop it spikes between 500ms to 800ms. On weekdays she has to shut her laptop off and put it in my mothers room at 9PM, by 10PM-11PM my ping returns to 60ms. My brother gets home at 3:30PM and when he is on it remains at 60ms. And if my sister doesn't use her laptop - say she is at a friends or something - my latency stays at 60ms until she gets home and uses it.

We have 6 pc's in our house + a ps3 (mine) and an xbox (brothers - we aren't very close either ... ), and my fathers television, that are all connected to the internet. One PC is only used for the printer - it is very old, don't know why we still have it - but it is rarely used. Basically I can have everything except my sisters laptop running and streaming, and my ping will be at like 90ms tops. But if it is just my pc, and my sisters laptop running, it sits at 500ms-800ms. It only started doing this when dad got her that laptop from Wal-Mart. And it's not just my pc it lags. And it's not just video games. When her laptop is on nothing streams properly including youtube and netflix, webpages load horrendously slow and usually have errors when they do, skype drops all the time, it's just an overall connection issue.

What is causing this? Is it ISP related and coincidental, or is there something distorting our connection in my sister laptop? It's not an urgent issue since we're moving on the 11th, and we'll know what it is shortly after then when we get our internet set up, I'm more just curious - and why I'm not asking our ISP (we'll be switching ISP's after we move). I'm posting this is wireless since I'm not sure where else, and most of our devices run wirelessly.