Oh, Nana69, my heart goes out to you and the child who is "picked on." Iv heard before of families where one child is the "Golden Child" and can "do no wrong," while another is the "scapegoat." So I guess this is not unusual, But it is still wrong, imo, and very sad.
Is it possible that the Golden Child is so good most of the time that dd looks the other way when they're not? Or that the scapegoat is especially difficult, argues more, or whatever? This might be a reason for the different treatment, even though that doesn't excuse it.
How about the other two kids? Even though they aren't scapegoated, they must be aware of the favoritism, and it must hurt them, too.
Not sure there's much you can do about this. Have you spoken to dd about what you've observed? Do the kids say anything?
If you've spoken to her to no avail, then I'm afraid all you can do is be as loving to all of them as you can, make sure they all know their talents, abilities, etc. and that they are worthy human beings no matter what. Please resist any temptation to balance things by favoring the scapegoat, as that would hurt the others and probably explode in your face, anyway. I wish I could think of some "magic" things you could do or say to fix this situation, but I'm afraid I can't. Perhaps someone else will.