It's great that she's good with other cats, but that is a problem from your angle. She can depend on your other cat for all interactions and may not warm up to you, I've heard it happen so I'd be proactive about this. I'd treat her like any regular feral cat you're trying to tame. I don't know how safe she feels in your overall house, but she doesn't feel safe with you, so that means she's always going to be a little on edge in your house. Maybe this would go away with time, like other suggestions, but I'd rather force the issue a bit more. Meaning I'd personally separate her from your other cat and put her in a "safe room" like a bedroom or something, a place you're willing to spend a lot of time with her. In that room she should have everything she needs, litterbox, food, water. The food you should bring to her at set times. She shouldn't leave that room until she's more comfortable with your presents.
She shouldn't have a place to hide from you that's extreme, like under a bed. Her hiding spot needs to be a place where you can reach and touch her (or in her thought process, harm her) at any time, but that you are not. This will start to sink in, that you're not harming her. Spend time in the room walking and moving around slowly, talk softly to her, pet her when she's eating (or whenever she'll allow), try playing interactive wand toy games with her, don't make eye contact with her (if they don't trust you they see this as a threat), sit on the ground a lot. I mean a lot. That way you're not so scary. Do something simple, totally ignoring the kitty, like reading (aloud would be good so she gets used to your voice) or bring in a laptop or something. Give her lots of treats, coaxing her closer to you. I'd bring your other cat in for visits only when you're there and only after she's made a little progress in trusting you.
I have a semi-feral cat in the garage, I spend an hour or two in their daily and he's been there for three weeks. He's gone from a cat that wouldn't come out of his hiding spot (on top of a fridge) to a cat that would come down growling, hissing, attacking and being food possessive... and then again to a cat that would sit beside me, and then a cat that's a moderate version of all those things (we're working on it), for the last week and a half he's been seeking out pets and is far less scared of me standing or walking about the room as he gets used to me. He even allows slight petting while he's eating - considering how food possessive he is that's a milestone
A lot of progress can be made in a short time if you focus on improving the problem, and that's going to be good for the cat and for you.