Looking for a phrase that involves comparison to overlooked stepsister

In certain places I have read a phrase that involves comparing someone/something to an overlooked stepsister (originating from Cinderella maybe). It’s supposed to convey the neglected status of that person/thing. Does anyone know what exactly the wording is and how it is properly used? I can’t seem to find it online (google takes it too literally) but I am sure I have heard it used. Thanks.