What does it mean when she said her children " are no cinderella?" Read below [I Love Family]

Im having some problems with my family. This week my cousin's mother made a status directed at me, saying I corrected her son about the word " family and relative " when he was 5, and I around 8 or 9. So I asked her about it, she brought up old hurt then I explained that I'm sorry if I offended her children with my outspoken nature as a child. She later started to apologise but in her long paragraphs she told her children that " they are no Cinderella children, because she got married before she had them with my uncle..., they are no Cinderella.." I left it there after we cleared up matters and didn't ask about that so not continue since she apologised. My mother mentioned that it's because I was born outside of marrriage ( something
the aunt has always looked down on her for) and they had a problem because my grandfather showed me more attention. It seems they have always felt left out but why the term Cinderella...??