Be nice to them, that will bewilder them.

or...scratch your balls.

Ah, I hate that. Haven't found a good response, either.

tell your step mom "You are a fat fuck go on a diet."

kill your step sister.

I definately vote with scratching your balls. Either that or walk into the room that they are in with your hand inside your pants, scratching away. Say nothing, until they say something. If they stare dumbfounded, say "oh well, that didn't work", and walk out!

After you take the photo, tell them that you're going to use it so you can practice making the same face and return the favor for 'em.