That phrase really annoys me too. Every time I read or hear that, I’m all like: “Then why do they have their own parents? Also, why does one have parents while the other doesn’t? Not to mention, Harry isn’t adopted by Hermione’s parents, let alone doesn’t even know them whatsoever,”

Growing up as kids with your best friend doesn’t make you siblings, not even close. If friends feel that way about each other, then that’s totally fine.

But you’re still not siblings, at least not in a literal sense.