The Masked Children

This story is about a sickly 15 year old boy named Yuma Kototsu who suffers from a disorder that weakens his immune system. He meets a girl named Sayuri Kogane who admire Yuma's selfless kindness and decides to form a friendship with him, alongside Yuma is his childhood friend Imasu Sonosuke and Yuma's older sister, Himari Kototsu who seem very resistant to letting Sayuri involve herself with Yuma.
Those masks that all 3 of them wore stood out to Sayuri, which after a lot of prying and a bit of trickery she finds out the truth about the masks that they wear so persistently.