I know a woman who recently met a half-brother she never knew she had, and her response to their first meeting was, "He was a very nice stranger." I said, "Yep, that's what he is to you." He knew he was adopted and didn't know he had a biological sister--but he had a sister already, and the genetic half-sister wasn't that person. She, in turn, didn't even know he existed. This has been the source of some weirdness, but not great social upheaval. If some evil sorcerer wanted to, I bet he could get this guy to scream, "Get away from my sister, you bastard!"--about his sister, his real sister, the one he spent his childhood with, not about my friend, who is...a very nice stranger to him, and ought not to be menaced by evil sorcerers, but.
And I have my own long-lost relative. My dad's father opted out of our lives for incredibly stupid reasons when I was 3. My dad reached out to him and got rebuffed, and we didn't skip family gatherings where he would be, but he did when he knew we'd be there. Then when I was 21, he wrote to me to tell me self-justifying, poorly constructed lies, some of which were easily externally verifiable as counterfactual. (Note to would-be liars: do not lie about things that are on a public record. It's insulting as well as dumb.) Here is how this works: your family is people you have actual relationships with. When you have declined a relationship? Not family. Relatives, possibly. But not family. So "You have to save him! he's your grandfather!" would have about as much meaning for me on this front as, "You have to save him! He's a fellow human being!"
So when long-lost relatives show up in books--when someone turns out to be someone else's ancestor or sibling or some other biological tie--I am not thrilled. I do not gasp with the shock of how powerful that is. I yawn. Or I roll my eyes.
I suspect that one of the things going on here is the same as one of the things that's producing all the sexual violence in the field that's making me read new SF braced for the worst: we somehow have the idea that violence by itself is not enough. It's not horrible enough if you-the-protagonist kill someone, it has to be a blood relative. But you know what? Killing people is pretty horrible. Or it's not awful enough if someone is in peril, it has to be sexually violent peril. Again: the peril. If you take it seriously, it's quite perilous enough. And when Ambrose Bierce had the million and one Civil War stories where someone turned out to be killing someone they knew...he recognized that your best friend or next-door neighbor could also be powerful. He recognized that relationship was important, and even in his gimmick stories, blood wasn't everything.
You know what I'd really like to see? I'd like to see a fantasy story where they assume that blood will work for something only to find that it's completely useless for people who don't particularly know each other or have a history together. "Now we will bind you by sacrificing...your father!" "Uh, dude, my father is the guy who raised me. This is the sperm donor. Now I will win." Or else I would like to see it in reverse, where sympathetic magic works along emotional/social bonds, so adopted siblings would work far better than biological siblings who didn't live together.
Mostly, though, I just find it boring. "I am your father, Luke!" was a line that had gotten into my brain before I remember it doing so--it's not a very interesting plot twist by now. Let it lie. Find something else.