Well if your son opens a Barbie I'd guess your daughter would steal it. But if your daughter opens a cool game, then what's to say she won't pout if brother steals it? Maybe if you clarify like this, and mention that the white elephant was going to be something fun to do the night before, then they should have no problem with it. Mention some ideas you had - a dvd, light-up cups, a funny gag gift (search "Fred and Friends" on Amazon), legos, a cupcake decorating set, etc......
My only concern with white elephant is that you will have few 'players' so I don't think it would be quite as fun as you anticipate. Not unless you have a few "funny" gifts (huge box of 24 rolls of toilet paper...) plus a few "great to steal" gift that everyone will want (iTunes gift card/Target gift card)...
Other ideas:
- Christmas Mad Libs
- Make a gingerbread house (or nativity scene) using fudge piping as glue. Here is a pic of the manger scene (that's coconut dyed yellow for hay) http://74.220.215.62/~firstci6/blog/wp-content/uploads/20...
- Pop Popcorn and watch The Christmas Story movie or any other flick
- Get to know Step-MIL and ask about her traditions - tell her yours for your family (ie, church service, lights, etc) and invite them. Who knows they may say yes!
- @Gigdet's ideas - make SMORES!