It's amazing how even babies seem to know who they can trust. I would use the fact that your a mom first. When my daughter was upset  with my grandson, the crying and all the baby stuff, I would remind her of what is to come. The teething, possible collic, the other stuff. That there is 18 yrs that is that child's. These are things young girls don't think about. I would say this in a none threatening way, as we want her to look at you as the replacement mom when necessary. I'd give her ways out of keeping him, use the post partum  depression , that she is so young to have so much responsibility. Fake it till you convince her that you really care about she feels. This is stuff that I learned when I was in college and worked with addicts and stuff.

I hope this gives you some ideas. I don't know where my cards and stuff is. I'm in the middle of a move and can't find anything. He is a sweet little thing,  put a circle of protection around him and things will be as they are suppose to be.  I will check in when I have time, I don't know when we'll have intetnet , we're going home so it's 7 hrs from here. Be glad when it's over. Take care.................Mystic