ipod touch version 5.1.1

i have a problem with my ipod touch it is an ipod touch 3rd generation ipod that is 8gb and it still is 4.2.1. i want to update it to the newest version (5.1.1) but every time i click on update on itunes it says that my ipod is updated to the newest version?? my sister has an ipod 3rd generation but hers is 32gb and i think that they count it as an iphone because the summary on itunes has the picture of an ipod touch 4th generation soo i am confused i dont know what i am suppose to do how do i get my ipod updated to the newest version if you have the same problem and found a way to solve it please help me by sharing your ideas that would be very helpful of you. Thanks!!🙂