Uh no. We have already plenty of stories with sisters-who-aren't-really-sisters, but it's still taboo for them to be in love and the way they relate to each other is not necessarily linked to their blood relation.
The basic plot wouldn't suffer that much if they weren't related by blood. Just the fact that they were raised together is enough.
And by the way, this story doesn't even really address the taboo. It stays comedic. Which is fine, but then what's the point of making them blood related? If there's an elephant in the room, I usually expect the author to address it at some point.
The fact that they are blood related adds a potential to the story that isn't realized.
But if this story satisfies you the way it is, you do you as well.
They're both girls. They being blood related or not isn't a issue at all.
Incest is taboo because of offspring.
It's also taboo because of the social imbalances and messiness of dating family, but that's the same if you're blood-related or adopted family. (Some of that doesn't apply for step-family though, but a break-up can still be messy.)
Anyways, incest is some people's thing, seemingly including Mochi Au Lait, so... that's why it is what it is, basically.