So.. I spent some time testing things out.
You CAN marry your children, but you can only always pick the first one in the list, and also only for the first one the other party offers. So you have to find spouses for whoever is listed above. If the opposite just offers an old spouse and you dont have someone old to take him/her out of the list, kill with console command. Its not optimal but that way you can keep your clan alive and carry on your savegame.