What a tragedy! If only your parents could see how kicking him out makes things so much worse! In effect, they would be encouraging them to "shack up" and advance their relationship a lot faster than it would otherwise. They have only been dating for two weeks! If your parents were to simply respectfully disagree, there is no telling if they would even be together in another 2 weeks, or 4 weeks, etc. and your brother could still be living at home.
If I were you, I would not let the girl friend stay with you. I think they both still have a lot of growing up to do. I also do not think I would advertise to your brother too much that he may stay with you either. I know you will take him in when the time comes, but it may encourage your brother to speed things up a bit on getting himself kicked out. (It sounds like that my be what he wanted in the first place by telling your mom.)
I understand why he is so eagar to leave his JW home environment, but it sounds like he is not quite financially ready to conquer the world on his own yet. Unfortunately, it is inevitable that he will lose the support of your parents by , but somehow I think money and maturity makes it a lot easier to leave.
Good luck!