The nuclear family is a post agriculture invention. The norm for farming families, which were the majority of families for the past thousand or so years, was to have your whole family in one house, with grandparents doing most of the child watching.
Nuclear, in this exact context, I had meant monogamous, specifically not multiple women per man. Grandparents being included is almost moot, but culturally were very important, everywhere/when.
Before agriculture, specialization / modern civilization, and the _medical revolution_, of course you were going to have multiple women - most died in childbirth, and the ones that lived were too dependent to stand up on jealousy grounds.
The nuclear family only became normative in the 50's in the US: https://en.wikipedia.org/wiki/Nuclear_family