Your default best friends are your siblings. You argue, make a lifelong vow to hate one another, and then spend an hour hanging out as if nothing had ever happened. Although we frequently think there is nothing we can do to drive our siblings past the point of no return, what would happen if that were to happen? The scenario in the story below is so severe that it may turn even the closest of friends into sworn enemies. So, OP over here graciously offered to arrange and plan her sister's weddin…