From defines.lua:
Code:DIVINE_BLOOD_FERTILITY_MULT = 0.25, -- Fertility multiplier in a religious close kin marriage
There's also another line in there that reduces the chance of inbred kids. These two lines should be read together: they just remove a lot of kids that would've died anyway. If this line is going to be changed, you should probably also change that line(and the one that increases lunatic chance)