Yeah, it is just like Lisa Kelly. At the top of this page her hair is pretty dark. Yet at most concerts I have attended it is lighter. As one lady said that was sitting beside me at a concert, "it's all in a bottle." But it would be interesting to know her natural color and Hayley's.
I think Lisa said one time that her hair is a really light brown, and the video that I saw of Haley's parent's and sisters being interviewed they all had whitish blond hair so maybe Haley is a natural blond. Not sure though.
I also saw Hayley's parents and brother and sister(Sophie) in a video and I was thinking the same thing you were. Is there another sister? As far as Lisa is concerned I know that her sister Helen has dark hair. I am not sure about her other siblings. If she comes back to Celtic Woman and I get to see her again at a Meet & Greet, I will ask her about her natural color. I suspect that I won't be the first to ask.