This is an antiquated adage . . . Used to hear it quite a bit with the person using a sardonic tone of voice.
Reference this url: World Wide Words: Red-headed stepchild.
" The meaning of red-headed stepchild is clear enough: it’s used to describe a person who is neglected, mistreated or unwanted. The evidence shows that it was originally American; it has spread not only to Canada but also to the UK, though it’s unusual here and almost always appears in writing by Americans."