Ping Problems

Hi there. My two brothers currently have a big problem. Since they are reallly into online games, they need a constantly good ping but sometimes their ping drops to about 100-200. The weird thing is that my own Ping stays constantly at about 20-30.
And as soon as they disconnect my Pc their ping gets better.

So the last few months the have constantly been blaming me for their bad ping to the point where I bought a programm to monitor my down and uploads.

The result was that I barely use anything.

Does anybody know what is causing this?

- We have a 11 Mbps connection
- We have 5 Pcs, alle connected via LAN (2 via a Devolvo adapter)
- We have a Fritz Box 7360 Router

I will provide any additional information you may need.
Thanks for reading :)
(And sorry for my english.It`s not my first language)