I would try and interact with her with the brother not being around. He may be startling her and with him getting the "aws, cheers, attention" from you guys, it may be making her nervous. (maybe have one of you take the boy to another room to explore for 20 minutes while the other plays with the girl).
Have you had them to the vet? If there is a medical issue going on, cats will hide and be very shy.
She could just have a shy personality or be especially weary of new people. I would try some alone time just with her to help build a bond. Try reinforcing human interaction with playtime and treats. Does she like to be petted?
One of my cats is a super shy guy and finally starting jumping on the couch for cuddle this year. He is 4. Sometimes it takes a while...