The incest thing could have been completely removed and the story would have remained the same, so it feels like adding an unnecessary ick factor.
I think them being sisters (rather than just friends) adds a bit to the surprise factor. And it makes it easier for the plot to happen (although it could have been roomates if this was with college students).
Also really, every single girl she picks up goes for the older sister immediately?
That got me too. I don't think everything would work out like that in real life, but that's why fiction exists, right? Also, it could be that Maria specifically targets girls with a history of cheating on other girls.
Actually I'm confused if they really are sisters or not. Like Maria says the incest thing is sick, even though she's obviously really into that other girl, so maybe that means it's just a type of roleplay?
I'm pretty sure Takemiya intended on them to be actual sisters. It's not that common for teenagers to live with an older girlfriend, and the two of them do sort of look alike (mostly hair color and type though, since there's not much more you can tell from manga). But you can read it as you like.