The French feminist & anthropologue "Françoise Héritier" has published great works on that topic. She has been studying one-two populations of North Africa for dozens of year.

Sexuality varies a lot. Even simply in Antique Rom in Europe, men and women could have sex with their slaves, sex between women wasn't "sex" at all and people didn't care (however women couldn't escape the patriarchal system, but lesbianism is a very modern concept). What happened in private was private, and references to male-on-male sex are many in the roman literature. There are poems, songs and famous people who were notorious for enjoying men's company.

(For example what is considered incest changes).

For example: in some places a widow will marry her step-brother. In other it is strictly forbidden for her to touch anyone from her step-family. Some families will inter-marry in the family close or large. Some places foreigners should not be touched, others they are welcome. Can step-sibling marry? Or kids who shared the same nursing breast?

What was that?

The primitive people's idea is that we are made of a "stuff", which comes from what we eat and get inside of us (logic). People made of the same material shouldn't have sex as they are family. Interpretation of how babies grow (does the 'stuff' come from the mother entirely? from the father, what relevant part?) and how our composition evolves vary. Some places if you had sex, you get some of the stuff of the man, so shouldn't have sex with his relatives. In many tribes, if you grow in a family, you are made of the same 'stuff' since you shared the same food.