Susan does shut up at Joker's words, although it's more out of shock at everything he says than out of any inclination to listen to him. He knows he's trying to negotiate here, he knows what's going on, but they don't have time for that, not when they need to save Lauren. He needs his brother. But apparently it doesn't matter, he's being ignored, and so instead he falls back once again, to where Lauren was taken under, before trying to at least see if he can dig him out. He knows it's useless, especially since there's magic involved, but what else can he do?