Weddingbee boards

Hm, that's tough.  Especially since his sister is in the wedding party, too.  I would talk to the parents and see what they say, but I see two ways to handle it.  1) Ask if they would mind if you put that he's your god son, since in practice you have that relationship with him anyway.  They (or you) might be very particular about this term, so it might not work, but they could also look at it as just like you can never have too many aunts and uncles, you can never have too many godparents, and for the sake of everyones feelings, it's fine.  2) How important is it to you to specifically mention that the flower girl is your god daughter?  You could put both of the children without titles centered at the bottom of your program, something like this.  That way, neither one has a title, they're just the kids listed at the bottom.

Best Man            Steve Smith         Brother of the groom

Maid of Honor     Jane Doe             Cousin of the bride

                   Flower Girl       Susie Johnson

                   Ring Bearer     Joey Johnson