...Now this takes us to the question of whether Worf had other siblings besides Kurn.
And that, too, is canonically unexplored. All we know is that Worf was the last survivor of his House, as far as he knew, until Kurn popped up. In theory, Worf might have had a whole horde of brothers and sisters who all perished either at Khitomer or then long before it, in the twists and turns of violent Klingon life. Yet their offspring might have survived the massacre just fine - but they wouldn't count as House of Mogh any longer.
Really, while Klingons by biology live long and thus might favor small families, Klingons as a culture live short and active lives - so one might expect them to breed like rabbits, and to start early. How many children would young Alexander already have in DS9 if he weren't so burdened by issues?
Then again, technically Alexander could have cousins (Klingon and human) from the mother's side as well, making the above statement a non-issue. And human (or other UFP species) cousins, from Worf's foster family's side, when we count second and third cousins.
Timo Saloniemi