Uchi no Ko wa Mahou Shoujo ni Naranai

A meteorite crashes into Earth, carrying the mysterious monster “Acrylamide” that threatens humanity. The only hope are the “Magical Girls” who have the power to fight Acrylamide! The high school boy Act supports his sister Shiyin, also known as Magical Girl Shiny, in her fight against the monsters, until one day when she is defeated and killed. Now what will Act do to get his sister back…? A time-traveling comic with a sis-con brother and law-breaking magical girl!

(Source: YK Ours, translated)