Internet and downloading is suddenly a lot slower.?

After my sister bought a Mac, the internet around the house got really slow for every computer including the MacBook. At first I tried turning off the router for 30 seconds and then plugging it back in and the internet went back to its normal speed for literally 15 seconds before it slowed down and dragged. The computer that I use is directly connected to the router and is dragging as well. I don't know if it is due to the MacBook or not but right after my sister turned off her MacBook the internet went back to how fast it usually was for about 30 seconds before it slowed down again. I don't know what the problem is but all I know is that an hour after my sister started using her MacBook yesterday, the internet started dragging for every computer in the house except for the short periods of time after I restarted the router and when she turned off her MacBook. Thanks for any and all help