Wow! I've never really given any thought to untame pets, but you know, it really is more common then you'd think! Here are some tips you can try!
1. Whenever you approach her, make it slow and easy. I see you've been doing that, good job! Just don't scare her at all! Be slow, even talk a bit gently to get her accustomed to your voice.
2. Cats can be jealous creatures, and often you can use this to your benefit. When she's around, stroke and pet your other cats openly. She will get hungry for the attention and come around quickly.
3. Buy some treats or a can of tuna! The best way to a man's heart is through his stomach, and it's the same for a cat. First, put some of the food in her dish where only she will get if. After doing this for a few days (along with her other food) start offering the treats or tuna only from your hand, to the point where she'll have to eat out of your hand or at least approach you for the food. Don't give up, and if she doesn't come the first time, don't just go back to putting the treats in her bowl. Keep offering it to her at different times or before you feed her and when you know she'll be hungry.
4. Be supremely good to her. Talk to her whenever you're around her or see her, blink slowly when you meet eyes with her (it show's that you're relaxed and non-threatening) and treat her kittenly gently to her face. If she knows that her kitten will trust you, she will, too.
Remember, this all takes time! No matter what you do, she'll grow tamer and tamer just living in your house. Try to win her over with as many treats and good food as you can straight from your hand whenever possible.
Good luck to you!
P.S. Pet her for me!