It's so awful that that poor little girl is pregnant and only a small baby herself. I know that you will do the right thing by sweetpea and take care of her and her babies. After all, imagine what would have happened if you hadn't taken her, they would've been born outside and who knows how she would have coped, it's a double edged sword really, but one I think you can handle. I'm sure you'll find loving homes for her babies if they are all okay.
I think she really needs your loving and support right now and I know you'll give her all you can, and that's just so wonderful that you've done this for her. It's best she's with you and not somebody else. She is safe and warm and happy and loved, and after all that's what all cats deserve.