Create a brand new holiday.  Think of food, activities, and decorations.

"Sisters' Day will be a day for everyone to celebrate with their sister.  And you get to go see a movie.  You get to go somewhere real fancy to eat.  Most important, you have to spend time with your sister, like Rebecca and Kate.  Then you can play a game out of your game closet, like Dominoes or Sorry, any game that you can do two people.  And the decorations are hearts with two sisters in the middle of them, and it says Sisters' Day.  You hang them all over the house, and then there are pink and red Christmas lights because you love your sister, cause it's Sisters' Day.  And they will have pink and purple sun umbrellas, and everyone will wear matching dresses, so no one gets separated.  Everyone has a fun time."