I remember watching the anime a few years ago and decided to read the manga to get a better understanding of the story. Holy fuck... it's terrible.
After reading it, I personally don't consider the main relationship to be *literal* incest because: 1) they're not biologically related, and 2) one of them started developing feelings before they even found out they were going to become stepsisters.
What really annoyed me was the constant "B-but we're sisters" dialogue. Like... no shit. They keep reminding the reader they're sisters, acknowledge that what they're doing is wrong, and then continue their relationship anyway.
It honestly feels like the author has an incest fetish but tried to tone it down by making them unrelated by blood. That's probably why so many people still call it incest. It doesn't help that there are actual biological sisters later in the story (side characters), and one of them has romantic feelings for the other.
I know the definition of incest can vary depending on the context. Some people only use it for blood relatives, while others include adoptive or legally recognized family relationships, where marriage or sexual relationships can still be prohibited even without a biological connection.
Overall, I think this story could've had a lot of potential if it didn't lean so heavily into the pseudo-incest theme. Then again, that theme is basically the entire foundation of the plot, so without it, there probably wouldn't even be a story.
Don't waste your time reading this slop lmao.