Dunno why it had to be a sister. It could have been a childhood friend fwiw. It would have been a bit less controversial.
I don't have anything against yuri incest when it's well done, but I can't see what purpose it serves here, except to show that "they are lesbian pervs anyway, no wonder they are incestuous too". And the colleague is totally understanding too. Yeah, right.
yeah the colleague was just so.... nonchalant about it all. the only conclusion I can make about the world they're in is that everyone's a gay, incestuous woman