@nwatn
That's a problem with how the manga portrayed the "isekai" part of the story.
Rather than say she was transported to a new world, it's better to say that she awoke memories of a past life when she was confronted by her brother and the other harem members from Yuri side.
Unlike other stories, she didn't took over the body of someone else; rather she's 100% Iris the noble daughter of a duke, that just so happens to have memories of a past life.
About the game, I dunno about other visual novel games but the ones I played you only had a very limited selection of dialogues that either push for the romance or not. My understanding of it is that precisely because of these limited dialogues that most anime based on visual novels have such generic main character with barely any personality.
So it makes sense that she doesn't know much about who Yuri is behind that mask of a girl in love.