People are predisposed to help family members, because that is one way to pass on a large fraction of their own genes. Just watch videos of people finding out they will be grandparents to see happy happy monkeys. However, we also have a "kick the kids out" phase starting with puberty and delayed by abilities still not equal to the available opportunities. The friction inspires more effort and learning.
However, there is also sibling rivalry, as each competes for parental resources, and, 10% of the time, a misidentified father in the mix. Even without infidelity, parents may divorce, fall ill, go missing to care for a grandparent, etc. Everyone is dealing with their own mix of arrested developments and phobias. The result is that many families eject members, and some fracture completely. Wills are often evidence of bizarre patterns.
I know a guy who lives on a farm that had its centennial this year. The family has grown quite large, so he had to spend a couple of weeks busily sprucing the place up for a big reunion. Then, he went camping to avoid it.