@Shewy92 It's called inseki, basically incest (brother-sister or father-daughter or mother-son) but the couple's not related by blood. It's a usual setup for hentai.
For real tho Japan is way too late on the gay marriage thing...
You woudo expect them to be one of the first to legalise it but they still didn't do it..
It's interesting that her mother doesn't even seem to be against same-sex marriage, just doesn't seem to think it feasible simply because the law forbids it.