Rant: Blood is thicker than water?

My writing for today is a rant, since this topic has been in my head for a while, and gaining momentum and trying to get out...

I see this too often on TV: Deadly enemies meet, there's an epic struggle, the good guy finally gets the upper hand, but just before he can strike, the bad guys says, "I'm your brother/sister/father/mother," and the good guy can't kill him.

WTF?? What has a little thing like shared DNA have to do with anything?

Now, if the writer tells us that in his world there's some sort of bond between twins that makes it impossible for one to kill the other, that sort of makes sense, but just barely. But if they aren't twins with the special bond, if they have never even met before, and all they share is a bit of DNA, why does that stop the hero from ridding the world of a great evil?

Where did this strange idea come from, that shared DNA somehow makes you obligated to the genetic relative, even though you have no actual family ties (have spent no time together, have not shared anything, etc.)?

As far as I'm concerned, people who share some DNA with you are only family if you actually have an emotional bond that keeps you together. A sperm donor is NOT a father; a parent is someone who is part of their children's lives.

So the bad guy revealing "I'm your father," should (imo) be met with, "No, you're not! I was raised by a peasant farmer. He's my father. He and I might not share any DNA, but he was a father to me, while you were off conquering the world without a second's thought about me. The only reason you brought up my origins is so I wouldn't kill you; if I wasn't standing over you with a knife at your throat, you wouldn't care that I'm the child of the girl you knocked up."

OK, maybe that's a little extreme, (for one thing, I certainly wouldn't want the hero to be monologuing while the villain is still alive and might take the opportunity to strike back...) but I honestly think that that's a much more appropriate reaction than "omg, you're my father? I can't kill you!" At which point the villain will probably say, "Ah, but I have no qualms about killing you."

Another pet peeve of mine: Many people use the expression "blood is thicker than water" when referring to this stupid shared-DNA thing. Ironically, the expression refers to just the opposite.

In this case, blood refers to being blood-brothers. I'm sure most of you are familiar with the ceremony of cutting hands and mingling the blood to create a blood-bond. Water refers to the family you were born into (as in the breaking of the water; coming from the same womb).

So the expression is saying that the people you choose to form bonds with are more important than the family you happened to be born into. I agree. I wish more writers would look at this, and stop using devices that make no sense.