Im starting to have suspicions that she was picked up by the Xuanyuan family, rather than born into it, especially since in one of the flashbacks she was thanking them for how much they've done for her and how she has to repay a debt. She might have a natual disposition to not be able to use magic much or at least not normal magic, so the Xuanyuan family either couldnt or wasnt able to teach her. Just a guess though, well see, cant wait for more!