Zte Zxv10 W300 / Pc On Wifi Network Makes All Others Lag?

wireless connection of my router, which is a ZTE ZXV10 W300 provided by my ISP. Me and my brother have our PCs on the same room and have decent signal at all times. The problem is that when my sister connects her netbook to the network wifi becomes unusable for us. I mean normally ping to the router from my pc is 1ms and when she connects it will get to the thousands or even time out. My sisters netbook has an atheros 9285 card, and the other two computer have a d-link n adapter, and a us robotics g adapter. I have tried multiple configurations such as b/g or b/g/n, with wpa or wpa2 but the problem persists. I checked my sisters netbook for viruses thouroughly, with malwarebytes, microsoft security essential +2-3 other antiviruses and none found anyhting. I also updated her driver to the latest i found. I am getting desperate here, and i do wish i wont have to buy another router. Just to clarify the problem is not internet speed, should be able to handle (and used to be) 5-6 computers at once, it is the connection to the router itself.