What's with the first and third one hanging out together today?

They aren't exactly the best of friends. The youngest is big and still like a baby, so he's always bothering his older sisters to play, and they freak out and beat him up lololol So I was so touched to see the first and third one hanging out together today that I snapped a pic... However, the second sister, who really, really hates the third one, has disappeared somewhere ㅠㅠ