I still find it a little weird that his sisters don't know Ibuki all that well. But you have them explaining their distance in middles school putting things into perspective. The youngest really likes her and it feels like she's been around. I re-read the story and it seems like Tomo an Ibuki became friends in their last year of elementary school so I guess it makes sense for his older sisters to not know her that well... Man these two really just hung out at school or went out.