Complex sentence: It was initially intended that the policy meant that only players born in the Basque Country could play for Athletic Bilbao, but this later extended to at least one Basque parent, and further into great-grandparents being sufficient to fulfill the policy.
Split into simpler sentences: It was initially intended that the policy meant that only players born in the Basque Country could play for Athletic Bilbao. However, this later extended to at least one Basque parent, and further into great-grandparents being sufficient to fulfill the policy.