Konosuba: An Explosion on This Wonderful World!, Vol. 2 (manga)

The ultimate magic. explosion. Pushing aside her hometown's teachings, Megumin, the genius mage of Crimson Magic Village, sets her sights on the raw destructive power of explosion magic. But when she arrives home with Yunyun to find Komekko, her little sister, surrounded by monsters, she's faced with a tough choice. Will she give up on explosion magic and spend her precious skill points on advanced magic in order to save her sister. And what has that strange cat got to do with all this.