i see what's going on here. He's just a replacement eh.

maybe not. nazuna might be an unreliable narrator and he's just feeding his guilt by punishing himself through having sex with his sister's bf. raws show he seems to have a thing for the bf. liking bf = betrayal. maybe matsuba, the bf, is indulging the dude even tho he knows it's wrong. he likes the gf (serina) to an extension, nazuna AS FAMILY but lusts for the guy so he's just as confused. he might be aware that nazuna's punishing himself. matsuba doesn't seem like the kind of dude who's use nazuna tbh.

i am tempted to mtl since we have the raws lol