Take it away and show the girl just what her hissy fit cost her? Oh, you bet. If she's that entitled and rude at 8, then I'd say she needs a little comeuppance. She's plenty old enough to say thank you and try to be gracious. And if she acts like that when someone gives her a gift--any gift--then I don't think it's at all inappropriate to say to her 'too bad, so sad' and let her stew in her own regrets.
One time when I was pretty little, maybe 6 or so, my sister wrapped up a present for me and put it under the Christmas tree. It was soft and felt like clothes were inside the package (no box). Getting clothes for a Christmas gift at that point in my life was just horrible. I remember telling my sister, glumly, that it was clothes and yuk, I don't want clothes, or something like that. She said "Are you SURE it's clothes?" The way she said it made me shut up about it, even though I couldn't imagine what the soft package could be other that the dreaded clothing, but I realized I didn't know, for sure, and she got me thinking. So on Christmas eve, I finally got to open the package. Did I find the dreaded clothes? NO!! I found a doll blanket! I learned a very important lesson by that--a gift is a gift, and be darn careful before you say you don't want something, b/c it may turn out that it's something that you really want after all.
But, I don't think I would have given the extra money to the brother and sister. That type of thing fosters competition and with the 8 year old being that entitled, I could see her destroying anything that she perceived her brother and sister got with 'her' money. Or just taking whatever from them b/c she perceives it as 'hers'. I would have just kept it to offset the greediness of the parents in dictating how much the uncle should spend on the children. And yup, if the parents raised that kid, then I wouldn't doubt that they did end up giving her the two tokens just to keep her from engaging in more entitled-princess-screaming-fits.