What does it mean by "there is no room for a scabby sheep in our pen"?

"Scabby sheep" refers to what McGinty's belief that Morris is disloyal.

“Well, that’s because he daren’t trust you. But in his heart he is not a loyal brother. We know that well. So we watch him and we wait for the time to admonish him. I’m thinking that the time is drawing near. There’s no room for scabby sheep in our pen. But if you keep company with a disloyal man, we might think that you were disloyal, too. See?”