I think of an 8yo I know, a friend's daughter. I love it but she (my pal) gets a lot of "oh, is that a family name...?"
The Sybil I know is fairly strong and not a girly little girl at all; she knows what she wants and she really suits her name. Of course there's no way you can know that, but if your child suited (for example) Olivia or Arabella then she wouldn't suit Sybil. Not much help that really.
I like Mabel too btw but I know I am in a minority 