After killing the red fleshy thing, pick up the gem it drops, and then exit via the emergency exit door in your inventory (click the door icon to make a door appear, then go through the door) - this turns the gem into a potion.
When you get home, give Imouto that potion (I don't remember if the cutscene triggers automatically, or if you have to have a drink with her through the interaction menu, as if you were sharing a carton of milk with her). After this, you'll be able to have interactions with her in the morning before going into the field by waking her up; currently just brushing teeth and then eating breakfast with her.