I agree you should let her move in but maybe you should get her on birth control JUST INCASE! You should let it be known the "NO SEX" rule still applies..but as others have said they are 16 and (maybe not at the moment but...) will become curious! Good luck and i commend you on taking in a CHILD that needs help!

It seems to me that this girl is moving in, not because she is neglected, but because she is your son's girlfriend and that is what he wants. As Jay said, if she were a 3 year old child, she wouldn't be moving in, you would be contacting someone to intervene.
If they are still virgins and they want to remain that way, it will not last long this way. Even if they want to wait, and from the sound of it, they feel that way, or at least she does by wearing a "purity ring"; you are putting temptation right in front of them. I am not being negative; I am telling you the truth. There will be problems, and don't be surprised if there is a baby on the way in the near future. It seemed to me that you were leaning more towards her not moving in from the start because you were unsure; something or someone changed your mind and it must have been a quick decision if she is already moving her stuff in since you posted yesterday.

I'm sorry, I saw that you did say he wears the "purity ring" too. But still, you cannot trust teenager bf/gf living in the same house to keep their purity. It is too tempting. And I am concerned about the example this is leaving for your daughters.

One more question: have you thought about what's going to happen if your son and his gf break up later on and she's living in your house?

Yes! They will have sex! What teenager dosen't? I don't think I would let my childs boyfried/girlfriend move in. Don't she have any aunts or cousins she could live with?