Note that this is true if you create the title, but it is not true if you usurp the title. (Usurped titles will normally revert to the default succession law - eg. ag-cog gav for feudal christians.) If a title already exists, it might be worthwhile to force it to be destroyed so you can recreate it, instead of direct usurpation.When you create a title, it receives the same succession law as your primary title. So if you adopt Primogeniture then create the Kingdom of Sicily, the kingdom receives Primogeniture.