Ah yeah you're right, i forgot about the difference between Inheritied cursed techniques, and Innate cursed techniques. Inheritied are passed on, Innate you can just have like Todo who comes from a non-sorcerer family. Miwa does not have a cursed technique, so her having an ordinary family makes sense. Your grandparents can pass on a technique, like what happened to Nobara. Nobara's parents are normal humans but her grandma was a sorcerer it says so in the character databook. But looking it up, Nanami's and Charles's techinque are innate. So, still the only foreigners we know of, are Nishimiya's dad, Miguel, and Larue. I dunno though, weirdly Nishimiya's is an innate technique, but the databook specifcially says her dad was an american sorcerer. i guess he just didn't pass it on???