Somehow I have a feeling that Lucifer will smite Cas and Crowley and they will end up in the Empty not the brothers. Plus, if I'm not mistaken there were rumors that Luci kills those close to the bros so yeah, that's what I think. If I think about it, Cas and Crowley are both at high risks, more than Mary because of reasons. I don't think Luci has a great interest in Mary. At least not at the moment.