As others have said, your house, your rules but having said that, we, all parents, want what is best for our children so it is hard to practice "tough love".
But having been there, it is the ONLY option. I have known quite a few teens (brothers, cousins, friends, etc) who lost a year of their life (during or right after high school) for exactly this reason(me included). Most of them managed to sort it out and go on to have great lives.
If the 18 year old moves out, where he "lands" is no longer your responsibility. Even if it is not good, I think, you need to let him make his own mistakes and face his own consequences.
I think you need to let your daughter know that there are rules and expectaions in your home and that if she does not want to "honor" them then she should be prepared to live somewhere else.
Then you spend many hours sending strongly worded prayers and pleas up into the heavens, hoping that she sees reason before she forces you to follow through. But be prepared, you may have to!