Hmm food for a bunch of 'around' 11 year olds...
If you are wanting it Japanese 'themed' but not necessarily traditional... Im just thinking of a few ideas...
You could go 'Hello Kitty' Crazy and have a Hello Kitty cake, and Hello Kitty decorations (I know that wont be hard to find!) or someother Japanese character... there isn't a shortage of them.
For food, you could go with western friendly california rolls... but you could put child friendly ingredients inside... pineapple and cream cheese is a goody! Karaage is yummy too, (crumbed fried chicken pieces) and you can put little sauces around...
You could get mixed rice... it has black rice in it, which makes the rest of the rice go a nice pink colour.
Onigiri is always good too, and often you can get little plastic shapes like hearts and animals which you can turn them into.
Hmm sounds like too much rice hahaha...
If you have a portable gas cooker you can have yaki niku cooking on the table.
Have a nice little pair of chopsticks for each person to take home with them afterwards, and maybe write all of their names in katakana as place settings.
Teach them how to make origami, or have an origami contest, to see who can make a crane the quickest, or something like that?
I dont know... im sure you will come up with some interesting ideas.
Maybe tell everybody to bring their slippers, so that they dont wear their shoes inside, and have everyone sitting on the floor.
Im sure your sister will love you for whatever you come up with anyway