Costume Ideas for siblings

[cshimmon]

Sorry, I'm sure there're a million threads about world book day, but I really struggling to find a costume for a brother and sister! The eldest is the girl, so things like Charlie and Lola are no good.

I'd be really grateful for any ideas at all!

[BlueChampagne]

How old? How elaborate do you want to get?

Thing 1 and Thing 2, or even the children, from The Cat in the Hat?
Peter Pan and Wendy?

[MumOfTheMoment]

Harry Potter and Hermoine Granger?