Koto-nee is totally out of her character with this confession stuff.
As far as I know, she's always been a goofy and lewd older sister that loved teasing her little brother. Nothing serious there. She even rarely spent her time with Eiji. Most of the chapters here were about Eiji, Nanako, Prez, and their shenanigans in the literature club.
So...yeah, it feels...weird for me to see Koto-nee become a hesitant maiden in love.
But still appreciate this arc because probably after Koto-nee graduate, her screen-time will be even lower than before (and also Prez, we might even rarely see her again after graduation).