Why does Bruno pretend to Gretel that Shmuel is his imaginary friend? Does she believe him? What do you think will happen if she finds out?

Bruno knows enough about Shmuel that he is not supposed to talk to him. He knows that is sister has bought into Nazi propaganda taught at school. Bruno knows that if Gretel ever found out that Bruno was talking with Shmuel that Shmuel would be in danger and he would never see Shmuel again.