From this year the third Sunday in June is the annual Brother's Day. Even a sister you fight with constantly has to do whatever her brother says that day, out of gratitude. So naturally, sex with your own sister is fair game. Even if your parents catch you having sex, instead of getting told off you get praised: “what a thoughtful, good sister.” The best possible day for settling every grudge you've built up against her. All that accumulated stress, unloaded at full force into her pussy on this one day a year. Because for this one day nobody can defy the brother — he's basically a king. Brother's Day is the best.