15! How on earth can you bring up and provide for a baby at 15- she is a kid herself.
I am nearly 21 and I wouldnt dream of putting myself in that position yet.
From what you have said so far, it sounds like she is having this baby to feel better about herself, and for someone to 'love her' unconditionally. Maybe you could offer to go to counselling with her and see if you can get to the root of her insecurities?

Need to go to work now, but Ill post more from there....