Gaming Ps3 Nat type 2, but sometimes type 3?

[gifi4]

OK, here's the deal. My sister has a laptop connected to a router and my ps3 is connected to the same router, I have set my internet up for ps3 and everything. I usually get Nat type 2, but when my sisters laptop is on (Nearly all the time) I get Nat type 3 and it disrupts some of my games! (mainly gta IV)

any help appreciated