It's actually not common knowledge! It has nothing to do with being smart and I myself know this bc of my immunology courses. I'll try to make it simple though. The whole concept of not getting transfusion from first-degree relative is that the donor's white blood cells will attack the recipient's tissues because they find it foreign and they think they should kill them!
If you get blood from strangers (Of course with the same blood type) your own White Blood Cells (WBCs) will find their WBCs foreign and since you have waaay more WBCs (and other immune responses and mechanisms) in your OWN body, your immune system will outnumber them and you(the recipient) are the winner.
As in the case of a first-degree relative with the same blood type and "other molecular similarities on your cells" your immune system won't find the donor's blood cells "foreign" and won't do anything (no attacking and stuff) but the donor's WBCs will attack your organs nontheless! And that's why it's quite fatal and it's not usually done...Hope it'd be helpful
WHY CANT THE BABY'S INTERMEDIATE FAMILY DONATE THE BLOOD? CAN SOMEONE EXPLAIN IM NOT SMART (╥﹏╥)