I'll pretend they are not blood related.
incest
...
Why you reading then?
Because this story is, above all, cute. These two girls are cute and their shyness is cute. And I like how they interact.
But incest
instead of pseudo incest
adds a layer of drama and taboo I find unneeded here.
Except...that's what the story is about. That's the basic plot and the reason the characters relate to each other the way they do. I imagine it must be difficult to enjoy the story while removing the story itself, but you do you I guess.
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.