my iphone 5 keeps dropping wifi connection repeatedly. HELP!!

ok so i got my tmobile iphone 5 on the 12th and so did my sister the same day. ever since then my wifi network is going haywire.

now when i am on my network alone (sister iphone 5 not connected to network) my wifi works flawlessly. though the bad part starts when me and my sister are home at the same time with both our iphone 5's connected.

i cant use any data heavy apps otherwise our whole network crashes and by crash i do mean the fact that every device in the house is kicked off and you have to wait five minutes for the wifi to come back. this happens as soon as i start using the wifi for apps like youtube or netflix more than just having the phone on stanby with it simply connected to wifi doing nothing.

like i said there are no problems when my iphone is the only iphone on the network.

im already on a replacement phone so i dont want to call for another replacement if the phone isnt the issue.

also the router we have is a linksys tmobile router WRTU54G with WPA personal security.

when the problem occurs at the time there are usually only 5-6 devices on the network including the phones and also an xbox 360, ipad 3, and occasionally a laptop or pc. this problem never occured when both me and my sister had samsung galaxy s 3's.

PLZ HELP!!!!