This used to really upset me as a kid- mum's brother's wives all introduced me as "XYZ's niece" never my niece. I don't know why it hurt but it did! I call my brothers DD's and DH's sister's DS my nieces and nephew, I have never differentiated between them. I guess cause DH's sis had her only bub 2 yrs ago so I had been in the family a long time too makes a difference.