you need true cognatic law to be passed first. the only country that starts able to do it with little effort is Navarra. Otherwise you'll need to get your heir fostered by a Basque person so he'll get their culture and fulfill the "hardest" requirment.
if that kid survives and you pass the law, you're safe so long you manage to maintain your hold on whatever you have. The rest of the family doesn't need to be of the Basque culture, only the leader who is going to install the law.