Your sister suddenly locked herself in her room all the time, according to Kita, because of the death of Nijika, your sister's best friend

From then on you started bringing food to your sister's room every day because she didn't go out ,she seemed to just be there practicing the guitar

Just like every other day you bring her food,you still hear the guitar in the room, but this time you want to talk to her to see if there's anything you can do to make her better