What is "real"...when it comes to being related?

I have written a series of essay/article type pieces about helping my adopted daughter find her birth relatives and how I am coping with her decision to live near them.
I just received a comment from a reader who mentioned that adoptees should find their birth relatives and being adopted means their identity is thrown away, so to speak.
Well, in a perfect world I suppose that everyone would be raised by their birth relatives; but a variety of circumstances seem to prevent this from occurring.
For example my son said to me today that my granddaughter told him he wasn't really her uncle, he wasn't a "real" relative. This bothered him. Yes, she is the daughter of my son's wife and her father is not my son, but she has been in our family for more years than she has not; and I have always considered her my granddaughter.
Hm...I suppose some might stick to the belief that the blood is what matters, the genes, the heritage.
But what about the years of raising someone, through tears, and fears, and laughter.
It is all complicated, sure, but it doesn't have to be...what is real? I say, LOVE...and that does not need blood...it just needs to be needed.