Where does a nun rank in the feudal system?

They were roughly in a middle class- neither commoners nor nobles. I should point out in some countries ( such as Austria) they had Princess- Abesses who were very much nuns- and of the Nobility. So there was some representation of the Noble classes. In Catholic Countries, Nuns often acted as ladies-in-waiting to Catholic Queens and princesses, and officiated at funerals,etc.