Ui Hirasawa is Yui's younger sister. But instead Ui acts more like the "older sister" of Yui, since she is more responsible and mature and takes care of her older sister. Yui is quite happy to be able to have Ui as a sister and loves her very much. This is proven when she decides to make a meal for Ui when she got sick taking care of her despite her lack of cooking skills compared to Ui. Yui even dedicatesone of the very first songs that she writes to Ui.