And there is also "temeljni zakon" (beside "sistemski zakon") in Angleško-slovenski pravni slovar as translation for "organic law".
In corpus (beta version of FidaPlus):
sistemski zakon - 1436 hits
temeljni zakon - 431 hits
organizacijski zakon - 3 hits
But I'm no expert in this area, so I don't know if these are synonyms or not.