By law the custody of the child automatically will go to her. Unless you can prove her unfit as a mother, your chances are slim. Also bare in mind if you prove her unfit then you are not next in-line. At that point anyone with an interst in raising the child is eligible and the child will be placed with the "best home"
Now on a more personal note. 15 year olds are capable of raising a child. And as her mother I would hope you would be concerned with HELPING HER BECOME A BETTER PARENT!,
So my personal opinion, invite her to move in with you and offer to HELP her raise the child, but remember she is the parent and gets to make the descisions. You should only over-rule her in cases where they would prove dangerous to the child.
