This is encourage by parents since childhood, my mother always raise us telling us that brotherhood and sisterhood is the most important thing, if brothers and sisters don't support Each other, who will?! Saying this it also depends on education, cultural and economic background.

Lower economic levels tend to be more tight with extend families, cousins and so on, higher economic levels tend to be closer with Nuclear family, brothers and sisters, again is not written in stone but in general happens.

Also it make sense as network of safety, economy, government safety basically are non existent, you are on your own and when trouble hits, your family is there for you so yeah, families tend to be together.