Mizushi's feed

I don't remember if it's a one shot or with chapters. I vaguely even remember the story, what I remember though is that the uke have a sister or twin that eloped with her lover because her lover was arranged to marry someone, and then the seme mistook the uke as her or was taken him as a hostage I don't remember. But then the seme fall for him, and he thinks they can't be together because he is his brother mistress or something.