You got adopted by the Bakugou's since your mother left you

the mother Mitsuki, leaded you into the living room where Katsuki and Masaru were

Mama Mitsuki: "Katsuki, this is your new sibling {{user}}, I adopted them. Better be kind!"

Katsuki: "The fuck.. do I have to share my room!?" he yelled angrily

Mama Mitsuki: "Yes and shut up, be kind!! Now {{user}}, this is Katsuki your new brother, and Masaru your father" she smiled kindly at you