There is one obvious reason Stephanie always loses and cheating is not one of those.
If you have taken microeconomics class in college, game theory is one of the important aspects of the course, which allows a person to strategically determine the choice with the best possible payoff given the possible risk/reward. In the game of rock, paper and scissors, Stephanie has to pick the best possible strategy (e.g. picking paper if Sora picks rock) to get the outcome she wants as picking the same or a move that will make her lose will give her the worst payoff. In game theory terms, the payoff matrix for this game is the following:
Note: I shared the correct explanation for what happened in Game Theory terms in the comments
From this table, she should have taken the risk and predicted that Sora will play rock based on his prediction that his opponent will choose scissors from an assumption that he will always play paper. If she taken the risk and played paper, she would have a better payoff opposed to losing. The problem I see here is that she thinks that game theory is just about the outcomes and not coming up with the best strategy. Not only that, she is unwilling to take the risk even with the possibility that he won’t pick paper, which is the main reason she always loses. This is also apparent when she knew that her opponent, Kurami was cheating after he told her, but didn’t speak up. That, and her defeatist mentality due to humans having no magic is no wonder why Imanity is in such a dire situation.
Game theory aside, I still think this episode was very enjoyable. Besides the crude humor, which gave me some mixed feelings, I still like Shiro and Sora quite a lot even if his wish made his sister become a little jealous. Since the two knows everything about the kingdom, their main goal now is to now challenge Kurami for the position as king so that they can finally put Inamity back on the right track. From this, I hope that Sephanie will learn a thing from the two so that she won’t end up being a loser at games for the rest of her life.