I remember reading the first chapter of this years ago, but nobody translated the rest. Is someone actually picking this up again, or just uploading that? Hopefully it's the former, Ninomiya Hikaru's works are actually super good.
@NoTofu: It's not like no society has ever handled this sort of thing. It's really not that confusing per se, but dropping it suddenly in the middle of the set of hundreds-if-not-thousands-of-years-old traditions of a formerly monogamous society does get confusing quickly.
(I haven't read this particular manga (and probably wont if it never gets picked up) but it's actually a pretty ripe topic for speculation. There isn't any single obvious reason why a society couldn't be structured in such a way, so it's an idea that's shown up in, e.g. the more thoughtful/speculative side of science fiction for quite some time.)
As for in-law chains in particular... wouldn't they really be no larger than they used to be (though they'd chain through spouses instead of just siblings)? You'd presumably just invite the most immediate set of in-laws to your parties, more or less, like normal.