Dhara gets crafty

My 14 years old was punished for an attitude issue for three weekends. I marked the weekends on the family calendar. Her 10 years old sister decided to play a trick on her and add an "X" on a fourth weekend then immediately erase it. She told her older sister that it looks like she (the 14 year old) had erased a date and might even get a fifth week added to the punishment.