You suggest a dip in the hot tub out back.

You get all attention back on you with a finger snap, like you've just remembered something.

"Hey guys, I just realized, the snowfall means this is a great time to enjoy the new hot tub! And if we eat first, we won't be able to use it all night. Do you all want to take a dip while we still have the chance?" The parents will only be around for this one night (at least that's what they think now...) and can see the logic in this. As you look around, though, you can see that some people are trying to hide their true feelings about the enterprise. Hot tubs mean swimsuits, and cold weather means erect nipples.

Come to think of it, nobody packed a swimsuit. After all, it's the middle of winter, and you haven't had this hot tub for very long, so the thought didn't cross your mind before. Your Mom voices the same concern to the room.

"Well", you offer, "this would be a great chance to really inaugurate the hot tub, since we're all here at the same time. I was hoping it could be a new winter tradition." You give a brief pause, giving the impression that you're mulling over something you actually decided on well in advance.

"We're all adults, right? Maybe we could just get naked? I mean, we're basically all family here." You see Sophia practically shaking in the corner, petrified at the prospect of sharing a hot tub naked with you, Emma, and your folks. Before she can let out a peep, you follow up and address her directly. The direct social pressure will get her to meekly go along with your ideas, you know this for a fact.

"And Sophia is a pretty free spirit, and you're one of Emma's best friends. You should join us. Right, it's a new progressive era. Communal nudity is a bonding ritual!" You're glad to have caught that special on the Discovery Channel about relationship customs in foreign countries, it's coming in handy as you blather and field questions. You can actually see a sense of relief overtake the room, as the parents realize this is the best excuse they'll have to see some naked people and let off a bit of stress.