[EmilytheStrange]

I'd get one of those big sibling shirts and make them wear it together. Or, alternatively, completely separate them where they're not allowed to be in the same room at the same time or something. The goal being to teach them to appreciate their sibling and playmate.

honestly, pretending to kick each other doesn't sound like something that needs a big, grand consequence. Actually hurting eachother would be different. Sounds like they're doing it mostly to annoy eachother and you. So, don't take the bait and make a big deal out of it. Just separate them. Or conjoin them.