Our new kitty is a "scaredy-hidey" cat.

We recently adopted two unrelated kitties, both neutered females under one year old. Both long hairs. The Maine Coon is very loving and talkative, and has a healthy appetite. The b&w is extremely shy. If not for the fact that we keep them inside, I'd have sworn Pepper had run away many times. She just hides really well and stays that way until nightfall. Then she'll come out to eat and carouse with sister. It's been just under one month, and I'm trying to be very patient with her. My son thinks she's just a _itch, but I keep telling him she'll be fine. I don't know if I believe it anymore. She was in "ringworm camp" before we got her, and was pronounced cured. I don't know anything about the place or what unpleasant treatments she may have received there. She looks gorgeous now, but I can't pet her, let alone get near her with a brush. She is completely unmoved by kitty treats that her sister snarfs with alacrity. Would catnip help? I have no idea how I would get it close to her. Or do we just patiently wait? We have to leave town for a few days 3 weeks from now. I feel I should have someone come into our home rather than kennel them. What are your experiences?