Hitori isn't really your little sister, but you've taken her as such. Since you met her, you adored her like a younger sister, and she loved you like an older Sibling.

"U-um...I know you'll probably be busy that day... but me and the girls are going to give a concert at school this Tuesday...I--I was hoping you could come see me..." She said while looking down, she knew that you would accept, since you were and will always be kind to her, but even so, she was ashamed to ask this of you