Relationships in this world are not just by blood. They are when two minds connect. When I can guess what is going on in the brain of the other. When I understand what the other person must be feeling.
These relationships are not friendships. Friendships have an element of 'commonness'. You are friends with the people in your class - people from school - people from the same building.
There are some relationships which extend beyond all boundaries and conditions. The attachments that these relationships bring are stronger and deeper than the blood that flows through my veins and the relationships it brings.
Attachments are like the rains. You don't know why they happen. But you can't not know that they are happening.
I am not undermining the importance of blood relationships. When a blood relationship is accompanied by a connection of the minds, it can certainly work wonders.
But at the end, attachments, unlike the human body, are persistent. That's another paradigm altogether though...