Someone using the name Doncuneo asked at reddit.com for responses from people who have consanguineous ancestry...
These things contributed to things like:
- Multiple siblings from one family marrying siblings (or other close relatives) from the same other family
- People being "double first cousins" and even marrying as such
- With or without her husband's knowledge, a wife getting pregnant by one of her husband's brothers or her own brothers since it wasn't happening with her husband
- Children and step-children "taking over" for their deceased parent in becoming the lover and/or breeder in the home
- Siblings experimenting in their youth and having spousal relationships as adults (parents died, they are left with the farm; "spinsters" and "confirmed bachelors" living together or with each other)
What all of this means is that actual family trees usually don't have as many branches as people might assume.