But why do you need access to any of your half sibling’s personal data to do that?

Nobody “needs” it, lol. People do it because it’s interesting to them. That’s why it’s opt-in.

Why do you need access to everyone who opted in’s data to do that?

Why does Facebook need to show you other people’s profiles and posts? Why does Lemmy show me your profile and posts? It’s how those services work, and people choose to use those services because they work that way.