When you say you value a 'nuclear family', instead of just 'family', the distinction means things like 'I do not want my parents to have more than a minimal role in their grandchildren's lives' and 'my siblings and their children are to be kept separate from my children'. That's what the 'nuclear' part means!
Mom died and now Dad's dropping hints about moving into your furnished basement? Sorry, we're a nuclear family. Going halves with your sister on a duplex with her and her family living next to you and yours so all the cousins can grow up together? Absolutely not nuclear.