My Polish grandmother called my aunt something like "poopka"
Two possibilities.
First, more likely, is that it's borrowed from German, in which 'doll' is 'Puppe' (from Latin pupa, young girl)
It would be Polonized as pupa (or more likely with the diminutive -ka as pupka)
Geerman loanwords are found throughout both standard Polish as well as many regional dialects.
Second, she was calling your aunt 'little butt'.... (pupa is an informal word for 'buttocks').