Sleek And Sly

Riddle: Once, there was a restaurant for foxes. A family of foxes went there. There were two sisters and one brother and of course, the father and mother. They were having lamb stew. After eating, the younger one of the two sisters needed to go to the bathroom. She went with her older sister to the bathroom. After a while, the sisters still didn't come back. The brother was asked to go to the bathroom and get them. It turned out that the younger sister was allergic to lamb and didn't want to make her mother panic. But, how did the brother manage to get into the bathroom to figure this out? It would take a long time to break open the door. And every other way seemed impossible. Or was it? Also, why weren't the sisters in the bathroom that he managed to get in the first time?