Another way of thinking about it is the age old saying:

'Blood is thicker than water' when referring to family over people you encounter.

Beyond the shared experiences and the irrational bond (I use the phrase as devil's advocate whilst being happily irrationally bonded with my family), there's not a lot to prescribing value to familial relationships over other ones.

We prescribe irrational value to diamonds, because offering a big rock to a bride in front of our family imbues value beyond the intrinsic worth of the rock.