Sibling Rivalry: the Beginnings

I've noticed a disturbing trend over the last few days... it is looking like the sibling rivalry is beginning. The kids are starting to figure out how to push each other's buttons. Ben pushes Becca's buttons by pretending to be a dog -- knowing very well that she does not like dogs. He gets down on all fours and sticks his tongue out and pants and chases her. A few minutes later Becca will get back at him by stealing something of his --preferably something soft, like a sock he's trying to put on, or a stuffed animal. She'll run away with it and shove it into her mouth and slobber all over it because she knows that Ben HATES it when she puts his things in her mouth. And before long they're both whining at me and I'm scolding Ben for scaring Becca and trying to pry a sock out of Becca's mouth. Ah, the things to come.