No, it was a trick!

Shiro can't bring herself to do it, and so exposes nothing to Sora. This way, they draw, and nobody has to do any forfeit of any kind.

At least, that would be the case, if her brother didn't know any better.

Sora has exposed a single butt cheek, also breaking their agreement. He has a knowing smirk on his face.

Shiro can't help but smile. "It was worth a shot."

"We know each other too well." Sora says simply.

"So, the game is decided, and I am the loser. That means I must give you a gold piece, and you must put the tip of one finger in my butthole." Shiro states flatly.

"But what if I don't? What happens then?"

"Probably the same thing that happened before. The universe just... makes it happen."

"Let's test."

Sora turns to leave Shiro's bedroom. As he does, a series of impossibly unlikely occurences happen all at once. A sudden powerful wind flows through the room, forcing both the door and window wide open. The door knocks Sora's arm, which sends him toppling backwards. He instinctively flails to try to avoid falling, and as he stumbles, the index finger of his left hand jams itself halfway up Shiro's butthole.

What? How?!

Because Shiro had been hit by the wind so hard that she had been blown face down onto her bed, her feet on the floor, her ass up in the air.

Completely impossible to happen under normal circumstances, but the rules of the universe, as set in stone by the Ten Covenants, dictated that the tip Sora's finger had to go up inside Shiro's butthole. And so that's what happened.

"Haaaah~" Shiro whines in discomfort at the feeling of her step-brother's digit burrowing beyond her tight sphincter. "This is so wrong!"

"You're telling me." Sora grumbles, removing the offending fingertip as gently as he can. He instinctively wants to wipe his finger on something in disgust, but he doesn't want to offend Shiro, and so he fights against the urge.

"It's okay." Shiro reassures him. "We can do this. Next test."