I know this person!
Two approaches I take - but first remember that you don't have to say anything unless they've Sadie something. "The look" only has the meaning we give it.
First is the proactive approach or the "before" approach. State very clearly what's going to happen and who is doing what - "Cathy is bringing this kind of cake because that's what Shirley wants. Can you be in charge of .....?"
Second is the "if you'd only told me" approach. "Oh, dear. Cathy went through all this trouble to get the cake Shirley wanted. If you had only said you wasted to help with the cake I would have asked the two of you to work together."
Big sisters and brothers sometimes like to throw their weight around. But your home/your party, your rules. You must be calm but firm.
Hope it helps a little.