I'll have to seek those bits of the anime out then... even if I decide to reject it, it's good to have para-canon.

I don't think she is, since it doesn't seem to be a four-person marriage. Admittedly, you're describing a situation that sounds emotionally incestuous to the point where a sedoretu would be a safer alternative just because it would clarify boundaries, but people are allowed to write badly or disturbingly if they want to.