Damn, Kurokawa-sensei got robbed of that w...though I wish we also got to hear more of Anri's occult spiel. The dedication is respectable! (applies to teach too with her not wanting a repeat of a student dying I guess)
Regarding the TL note: yeah, cousin marriage has always been a thing everywhere (though it's steeply declined in post-war Japan) and iirc it's legal there to marry a sibling who doesn't have direct blood relation (adopted / step).