Relationship between Tom and Meg (last book spoilers)
I always wondered about this in the series.
So if I understand it right, Zenobia the ex-girlfriend of Zeus gave birth to the first lamia demon at some point. This unnamed lamia produced the other lamias.
Zenobia, much later in life, also gave birth to Tom and his six brothers.
So the first lamia is Tom's older (half) sister, and her children, grandchildren, etc. are Tom's nieces.
That means, technically, Meg is Tom's great- great- great- great- great (x1000) grand niece since she is descended from/is the great- grand child of Tom's sister.
Instead of calling him 'Billy', she should just call him (Great) Uncle Tom.
(Or maybe they're just distant cousins...?)