Lois Wickstrom 
Super Enemies [EPUB ebook] 

Mira and Carl are enemies. They are also sister and brother.

They never want to do the same thing. Except, they both want to sit behind Daddy when they ride in the car. One day, Dad takes them to the beach. Carl is going to be the super hero and build a castle. Mira will be the super villain and destroy it. So, Carl builds better and stronger castles. And Mira comes up with more ingenious ways to destroy them.

Can being enemies be so much fun that it bonds them together?