
You're not alone, I've thought this exact same thing multiple times
I've even gone back and reread earlier sections trying to see if her being blind was mentioned but never was and in later chapters it's obvious she can see so I'm like..... I don't understand
I can only assume that since her and her sister look a like that's the main way to tell them apart maybe? But even then they don't look so similar that they're indistinguishable ╮( ̄▽ ̄)╭

Maybe it's to indicate more of her personality. All the characters I can think of in manga and anime that have their eyes closed usually come off as aloof or a bit of a prankster without malicious intent. Or it might be because she's actually looking down/through her eyelashes, which is submissive behavior that would fit with her circumstances.
Why does the author draw her with her eyes closed? I know she’s not blind, because I seen her open her eyes before, but the way she’s drawn, I keep thinking she’s blind. Lol
It was just a side observation