Well, traditionally 7th sons of 7th sons are supposed to be witches or at least have some magical powers.

Here's a quote I had lying about somewhere (don't know the source):
Belief in the special gifts of a Seventh Son or Daughter
can be found in Folklore throughout the world. Leslie
Shepard in his 'Encyclopedia of Occultism and
parapsychology' states that in Romania the Seventh
Child is fated to become a vampire. And that in
England in the seventheenth century there were
reports of a Seventh Son healing the deaf blind and
lame.
In France, The Seventh Son was called a 'Marcou' and
branded with a 'fleur-de-lis' {reknown worldwide as a
symbol of the Divine & Quality, and all that is Holy,
Click this Link for history}. Good Luck was also said
to attend the Seventh Child. In olden days, when
Captains of Merchant Ships were setting sail on long
voyages, they would try, 'often by force if
neccessary' to have a Seventh Son on ship as Legend
has it that the Ship wouldn't sink with a Seventh Son
on Board
The reason why the Seventh Child as opposed to the first
or the fifth for example-should have 'Special Powers'
has it's roots in ancient beliefs. The number Seven
has many mystical and religious associations. Apart
from the Bible telling of God creating the world in
Seven Days, The number features throughout the old
and new testaments.
Seven is a very significant number in numerology and is
the basic unit of transition in natural cycles from
the week as a phase of the lunar cycle to the idea
that many of life's patterns are divided in phases of
Seven.

Letty