As a godparent of four, it has been explained to me that not only am I to give "direction", but, I will answer before God, if my godchildren do not grow into good Orthodox Christians. Apparently, I have a bigger responsibility than their biological mother, in raising them Orthodox.
It's an awesome responsibility, and often leaves me rather concerned and worried....because it's harder and harder to raise kids Orthodox in such an anti-Christian (yes, I mean anti) environment.
As to your question Trevor, I can never marry any of the god-fathers, nor any of my godchildren.
I think you only "answer" to your godparents....and not to any of their relatives or other godchildren of theirs.