I would buy a gift for the 8yo and a litle something for her sister. My thoughts are set a budget and then divided it accordingly. I agree with BONKERZ..... crafty things are always good as I find most people will not give these type of gifts (prefering figures, games or dvds!).
Anyway, don't worry too much, what will happen is the birthday girl will tear open the gift say how lovely it is then quickly move onto the next!!!!!
Happy present hunting....