My husbands grand parents from both sides was born in The Netherlands. His parents both was born in The Netherlands. They immigrated to South Africa just after WWII and started a family in South Africa. They kept their Netherlands Citizenship till all 4 children was born. Then they both Naturalised. So after that came my husband. As a surprise though.. and we want to know if he can claim NL citizenship after his parents both naturalised. Both of his parens passed away. We want to know even though they naturalised if we can claim dutch citizenship for him. Maybe through his grand parents? My husband is born in 1969.
You lose the Dutch nationality in the following situations:
You voluntarily take another nationality. By voluntarily taking another nationality you will automatically lose your Dutch nationality.
This applies to your in-laws, as they voluntarily took South African nationality in 1966.
As such then they were no longer Dutch citizens when your husband was born in 1969. Therefore he does not qualify for NL citizenship. He can not qualify via Grandparents, only parents. (And if born before 1985 only via the father).