Hm, supposed there is a seventh son around who just isn't mentioned?
Or there has been a seventh son who died a short time after Esk has been born?
(If I remember right at her birth day it is mentioned that there are seven sons and Esk is the 8th child).
Not mentioned, too?
Okay, being fan I am able to interpret it in every way that it will fit.
And yes, these presumbly are some plotholes I until now haven't noticed.
But I have to confess although I like this book I seldom reread it, even if I reread the witch series I usually leave it out and begin with Wyrd Sisters.