I hope someone has some good advice!

I adopted 2 "rescue kitties" about 2 months ago...long story short, the younger of the two seems absolutely terrified of me. She was a stray kitten found in a parking lot...and when I rescued her she was somewhat tentative but she seems to be more afraid of me now than initially. She adores "her big sister" (ii.e. is comfy around the other cat)...loud noises (like sirens and stuff ) don't bother her so she isn't a "scaredy cat" in general but people really scare her and she hides under the bed most of the time that I am in the house. It breaks my heart that she doesn't "Know" that she will ALWAYS be safe and loved with me...any thoughts or success stories using HUMANE/GENTLE behavior mod or botanicals or meds would be appreciated. THANKS :-)