You're a love for taking them in and providing them a loving home!
At this point, I think the best thing to do is just ignore her. Since trying to bring her out of her shell hasn't worked, try the opposite. I wouldn't reconfine her - I agree, that would just stress her out.
Just think of Symone as a rescued cat, and you're her caretaker. She has food, water, a roof over her head and medical care when she needs it.
Mary Anne (hissy), who has been rescuing for over a decade, posted this in the stickied thread up at the top of this forum (Socializing a Feral, the Story of Lucky):
You asked if I remembered my first feral. I do, and it was many years and over 500 ferals ago. There were actually two of them, just kittens. I spotted them one night outside my kitchen window out in a field across the way a bit. They were just tiny specks in the distance, hard to distinguish exactly what they were. When I finally saw they were kittens I went outside with some food and as I approached, they scattered. I lugged a rock out from behind a tree, and used that as a plate for their food. I began to feed them every day three times a day at scheduled times. Sundance was an orange male and Cassidy was a female. The big movie box office hit at the time was Butch Cassidy and the Sundance Kid, so that is how they got their names. I left water and food for them as they looked like they had just left mom not to recently. Never did see another cat around them, so chances were good that mom was dead.
When the rains started, I piled a bunch of rocks together and made them a makeshift cave to sleep in and get out of the wet. Over time, and I know you want to know how long- but honestly, I cannot remember, it was months though. I gradually moved their food rock closer and closer to my kitchen window. When I got them so close I could reach my arm out the window to touch them. I removed the screen from the window and started leaving it open so they could come inside. I put dishes of tasty food on the counter at night, and when I woke up in the mornings the food was always gone.
One day I came home to find both of them exploring the house. I closed the window trapping them inside and thinking "Ah ha now I have two wonderful kitties!" LOL I couldn't of been more wrong! The minute they saw their escape hatch was gone, Cassidy scrambled up the wall and vanished into the rafters and Sundancer slid under the stereo table. We lived uneasily like that for weeks. They would only come out late at night, and not being litterbox trained, they left their piles everywhere. It was a real learning experience for all of us.
Eventually, they came to trust me, though Cass lived up in those rafters so long I was afraid she was going to grow wings and fly off. But with patience and love, they learned they were in a good place and I finally just learned to ignore them, but to see to their creature comforts. It was hard to do, because I wanted lap kitties, but once I started to pretend they were not around- and by that I mean stopped actively looking for them, or trying to coax them out, they relaxed and finally came to me. They were the best kitties after that and they lived a long time in great comfort and helped me with the others that came after them. A real baptism of fire that time was. I made a lot of mistakes and learned from them. They were great teachers.
Maybe this is what Symone needs. Simply - to be left so alone SHE decides to seek you out. The issue here is that you know nothing about their backgrounds. Maybe Symone was traumatized by humans in a way that Chantel wasn't - even if they're sisters. Whatever the reason, you're an angel for loving them so.