Wow, how sweet of you to take in this little cat family and take care of them. I also have a girl who was completely feral and over the past (almost) two years, she has blossomed into something I never imagined she would. The main thing is PATIENCE with her. Never make a move toward her, always let her approach you at her terms. That is what I did with Arwen. She was a cat that lived in my neighborhood for years, the neighbors guess she has been here for over 5 years, mostly eating from trash cans on garbage days, otherwise you did not see her, and if she did see a human, she ran. She finally worked up the nerve to eat some food I left her one day, and it turned into her returning daily for her food, and after months, she brushed up against my leg, but she would hiss if I made a move to pet her. After almost 6-8 months, I could pet her head a bit. Now two years later......I can pet her all over, and she even now allows my husband to pet her the same as me, and one neighbor lady that lives near me, she will also allow to pet her. It is amazing. She is already fixed, so she can't have kittens, and her ear is tipped....something that was already done when I found her.
I would highly recommend you finding a way to trap this cat and get her spayed, otherwise you are going to end up with a continuing pet population that will grow and grow. Maybe you can befriend someone at a cat rescue group, who can help you trap her? If you are successful in trapping her, have her ear tipped, so all will know that she is spayed.
I think you are doing a great thing. You have a nice outside place that you can make a place for her to get shelter. I'm not sure where you are, but if it is very cold, people here can tell you how to make a feral shelter with some hay and a rubbermaid container, or it sounds like you may have a nice place she can have shelter in during the winter. She will come around, I promise you, but it takes so much time and patience, and never, ever rush it. Even now, I can pick Arwen's front legs up, but I cannot lift her up off the ground, she will freak and run, so there is only so much she will allow. Your feral kitty may never allow you to touch her, or she could come around to where you can pick her up, but just don't try that or rush it with her, you could end up VERY hurt. Just be patient and she will come around, and she will love you for taking care of her.