One of the very long term effects of the Northern European style nuclear family structure where the eldest son isn't guaranteed to get the inheritance is that people are constantly moving and so you're going to have the least amount of inbreeding possible for a population. To make it and become worthy of starting a nuclear family, men will leave their households, their neighborhoods, their cities, their provinces, even their countries, and start up new families, and so there's the highest likelihood for two people who start a nuclear family to be unrelated.

The worst family structure for inbreeding would be endogenous clan structure (multi family household and cousin marriage is allowed), since you're going to be part of a family that has been there for a long time and you'll be interacting with other families who have been there a long time. This would include orthodox Jewish civilization as well as Muslim civilization. Inbreeding starts immediately, and by design. Royalty is similar to this as well.

The next worst would be exogenous clan structure (multi family household and cousin marriage is not allowed) since you would be trying not to marry your cousins, but you'd still be tied to the local geography and so a few generations down the line you'd be mixing bloodlines whether you like it or not.

One important thing is that the world today only tends to think about one goal at a time, and the world isn't so simple. Inbreeding is one thing to be aware of, as well as levels of innovation and general economic prosperity (people point out the higher inherent wealth in sharing a household, but by staying where family is instead of going where opportunity lives, overall family wealth will improve), societal stability and others.