Law / Where to get certificate of family composition (gezinssamenstelling attest) in Poland? [2]

hello there ..could some advice me from where i can get family composition certificate in poland ..i am a eu citizen who was living in poland (lodz ) from sept 2016 untill march 2018 with my mother ( she was living as a dependent of a eu citizen &holdig a residence card of a dependent of eu citizen )..now when i am beck in my home state (belgium )& want to register my mother in belgium ..& belgian city council ask me to bring certificate of family composition (attest van gezinssamentelling) from poland where i lived with my mother in same house hold ....no one seems no one in polish city council seems to know this paper ..thnx in advance bopprai